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/post.go

@@ -7,7 +7,6 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
-	"regexp"
 	"sort"
 	"strings"
 	"unicode/utf8"
@@ -343,20 +342,8 @@ func PostPatchFromJson(data io.Reader) *PostPatch {
 	return &post
 }
 
-var channelMentionRegexp = regexp.MustCompile(`\B~[a-zA-Z0-9\-_]+`)
-
-func (o *Post) ChannelMentions() (names []string) {
-	if strings.Contains(o.Message, "~") {
-		alreadyMentioned := make(map[string]bool)
-		for _, match := range channelMentionRegexp.FindAllString(o.Message, -1) {
-			name := match[1:]
-			if !alreadyMentioned[name] {
-				names = append(names, name)
-				alreadyMentioned[name] = true
-			}
-		}
-	}
-	return
+func (o *Post) ChannelMentions() []string {
+	return ChannelMentions(o.Message)
 }
 
 func (r *PostActionIntegrationRequest) ToJson() string {