[MM-22622] Add Ephemeral response when using mentions without permissions (#13902)

* MM-22282 Add Ephemeral response when using mentions without permission and add new prop to disable mention highlights on client

* MM-22622 Make test name test actually what it does and fix comment style

* MM-22622 Check ephemeral post created when post create with mentions on API

* MM-22622 More tests for App>CreatePost

* MM-22622 Make DisableMentionHighlights more concise and rename ephemeral post

* MM-22622 Dont send ephemeral message for system message created by user

* Trigger build

model/post.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	"regexp"
 	"sort"
 	"strings"
 	"unicode/utf8"
@@ -58,6 +59,8 @@ const (
 	POST_PROPS_DELETE_BY           = "deleteBy"
 	POST_PROPS_OVERRIDE_ICON_URL   = "override_icon_url"
 	POST_PROPS_OVERRIDE_ICON_EMOJI = "override_icon_emoji"
+
+	POST_PROPS_MENTION_HIGHLIGHT_DISABLED = "mentionHighlightDisabled"
 )
 
 type Post struct {
@@ -432,6 +435,34 @@ func (o *Post) ChannelMentions() []string {
 	return ChannelMentions(o.Message)
 }
 
+// DisableMentionHighlights disables a posts mention highlighting and returns the first channel mention that was present in the message.
+func (o *Post) DisableMentionHighlights() string {
+	mention, hasMentions := findAtChannelMention(o.Message)
+	if hasMentions {
+		o.AddProp(POST_PROPS_MENTION_HIGHLIGHT_DISABLED, true)
+	}
+	return mention
+}
+
+// DisableMentionHighlights disables mention highlighting for a post patch if required.
+func (o *PostPatch) DisableMentionHighlights() {
+	if _, hasMentions := findAtChannelMention(*o.Message); hasMentions {
+		if o.Props == nil {
+			o.Props = &StringInterface{}
+		}
+		(*o.Props)[POST_PROPS_MENTION_HIGHLIGHT_DISABLED] = true
+	}
+}
+
+func findAtChannelMention(message string) (mention string, found bool) {
+	re := regexp.MustCompile(`(?i)\B@(channel|all|here)\b`)
+	matched := re.FindStringSubmatch(message)
+	if found = (len(matched) > 0); found {
+		mention = strings.ToLower(matched[0])
+	}
+	return
+}
+
 func (o *Post) Attachments() []*SlackAttachment {
 	if attachments, ok := o.Props["attachments"].([]*SlackAttachment); ok {
 		return attachments