MM-21987 Resolve mentions in slash commands (#13762)

* Create infrastructure to manage mentions

Two new files have been added (along with their tests); namely:

- model/at_mentions.go: utilities to parse and manage mentions; for the moment,
it just contains a regex and a couple of functions to parse possible mentions
and to post-process them, but it can be extended in the future.
- model/mention_map.go: it contains two new types (UserMentionMap and
ChannelMentionMap) that both have FromURLValues and ToURLValues. These types
can be used when adding the mentions to the payload of the plugin slash
commands.

* Extend custom commands payload with mentions

Two couples of new fields are added to the payload; namely:

- user_mentions and user_mentions_ids: two aligned arrays of the same length
containing all the different @-mentions found in the command: the i-th element
of user_mentions_ids is the user identifier of the i-th element of
user_mentions.
- channel_mentions and channel_mentions_ids: two aligned arrays of the same
length containing all the different ~-mentions found in the command: the i-th
element of channel_mentions_ids is the channel identifier of the i-th element
of channel_mentions.

* Fix shadowing of variables and redundant return

* Fix shadowing of variable

* Address review comments (HT @lieut-data)

- Improvements in mentionsToTeamMembers and mentionsToPublicChannels:
	- Scope implementation details inside the functions.
	- Improve goroutines synchronization by using a sync.WaitGroup.
	- Retry lookup of username only if the returned error is http.StatusCode,
	  so we can return early if the error is more severe.
- Invert check in PossibleAtMentions to improve readability.
- Make user and channel mention keys private to the module.
- Allow the specification of an empty map of mentions in
(Channel|User)MentionsFromURLValues when both mentions keys are absent.
- Replace custom functions in tests with require.Equal on maps.

* Test functions to parse mentions from messages

* Extend plugin commands payload with mentions

* Add functions to CommandArgs to add mentions

The functions make sure that the maps are initialized before adding any value.

* Address review comments (HT @lieut-data)

- Adds a mlog.Warn to avoid burying the error when the user is not found.
- Improve readability in loop populating the mention map by moving the
initialization of the map closer to the loop and by iterating over the channel
itself, not over its length.

* File was not gofmt-ed with -s

* Close channel when all goroutines are finished

* Again, all code should be checked with gofmt -s

* Refactor code out of a goroutine

This change helps improve the readability of the code and does not affect its
overall performance. Less complexity is always better.

* Close channel and iterate over its range

Adapt mentionsToPublicChannels to have the same structure in the management
of the mentions channel as in mentionsToTeamMembers.

* Adapt mentionsToTeamMembers to new App

Commit 17523fa changed the App structure, making the *Server field
private, which is now accessed through the Srv() function.

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

model/command_args.go

@@ -11,16 +11,18 @@ import (
 )
 
 type CommandArgs struct {
-	UserId    string               `json:"user_id"`
-	ChannelId string               `json:"channel_id"`
-	TeamId    string               `json:"team_id"`
-	RootId    string               `json:"root_id"`
-	ParentId  string               `json:"parent_id"`
-	TriggerId string               `json:"trigger_id,omitempty"`
-	Command   string               `json:"command"`
-	SiteURL   string               `json:"-"`
-	T         goi18n.TranslateFunc `json:"-"`
-	Session   Session              `json:"-"`
+	UserId          string               `json:"user_id"`
+	ChannelId       string               `json:"channel_id"`
+	TeamId          string               `json:"team_id"`
+	RootId          string               `json:"root_id"`
+	ParentId        string               `json:"parent_id"`
+	TriggerId       string               `json:"trigger_id,omitempty"`
+	Command         string               `json:"command"`
+	SiteURL         string               `json:"-"`
+	T               goi18n.TranslateFunc `json:"-"`
+	Session         Session              `json:"-"`
+	UserMentions    UserMentionMap       `json:"-"`
+	ChannelMentions ChannelMentionMap    `json:"-"`
 }
 
 func (o *CommandArgs) ToJson() string {
@@ -33,3 +35,23 @@ func CommandArgsFromJson(data io.Reader) *CommandArgs {
 	json.NewDecoder(data).Decode(&o)
 	return o
 }
+
+// AddUserMention adds or overrides an entry in UserMentions with name username
+// and identifier userId
+func (o *CommandArgs) AddUserMention(username, userId string) {
+	if o.UserMentions == nil {
+		o.UserMentions = make(UserMentionMap)
+	}
+
+	o.UserMentions[username] = userId
+}
+
+// AddChannelMention adds or overrides an entry in ChannelMentions with name
+// channelName and identifier channelId
+func (o *CommandArgs) AddChannelMention(channelName, channelId string) {
+	if o.ChannelMentions == nil {
+		o.ChannelMentions = make(ChannelMentionMap)
+	}
+
+	o.ChannelMentions[channelName] = channelId
+}