MM-10833: send down computed channel props (#8953)

* MM-10833: send down computed channel props

This allows channel headers to reference channel mentions for a client
that doesn't already know about the channels in question.

We intentionally don't send down the props for the autocomplete and
search endpoints since they aren't used in that context, and would add
unnecessary overhead.

* update channel props on patch

* revert to treating channel purpose as plaintext

model/channel.go

@@ -32,21 +32,22 @@ const (
 )
 
 type Channel struct {
-	Id            string  `json:"id"`
-	CreateAt      int64   `json:"create_at"`
-	UpdateAt      int64   `json:"update_at"`
-	DeleteAt      int64   `json:"delete_at"`
-	TeamId        string  `json:"team_id"`
-	Type          string  `json:"type"`
-	DisplayName   string  `json:"display_name"`
-	Name          string  `json:"name"`
-	Header        string  `json:"header"`
-	Purpose       string  `json:"purpose"`
-	LastPostAt    int64   `json:"last_post_at"`
-	TotalMsgCount int64   `json:"total_msg_count"`
-	ExtraUpdateAt int64   `json:"extra_update_at"`
-	CreatorId     string  `json:"creator_id"`
-	SchemeId      *string `json:"scheme_id"`
+	Id            string                 `json:"id"`
+	CreateAt      int64                  `json:"create_at"`
+	UpdateAt      int64                  `json:"update_at"`
+	DeleteAt      int64                  `json:"delete_at"`
+	TeamId        string                 `json:"team_id"`
+	Type          string                 `json:"type"`
+	DisplayName   string                 `json:"display_name"`
+	Name          string                 `json:"name"`
+	Header        string                 `json:"header"`
+	Purpose       string                 `json:"purpose"`
+	LastPostAt    int64                  `json:"last_post_at"`
+	TotalMsgCount int64                  `json:"total_msg_count"`
+	ExtraUpdateAt int64                  `json:"extra_update_at"`
+	CreatorId     string                 `json:"creator_id"`
+	SchemeId      *string                `json:"scheme_id"`
+	Props         map[string]interface{} `json:"props" db:"-"`
 }
 
 type ChannelPatch struct {
@@ -163,6 +164,18 @@ func (o *Channel) Patch(patch *ChannelPatch) {
 	}
 }
 
+func (o *Channel) MakeNonNil() {
+	if o.Props == nil {
+		o.Props = make(map[string]interface{})
+	}
+}
+
+func (o *Channel) AddProp(key string, value interface{}) {
+	o.MakeNonNil()
+
+	o.Props[key] = value
+}
+
 func GetDMNameFromIds(userId1, userId2 string) string {
 	if userId1 > userId2 {
 		return userId2 + "__" + userId1