PLT-5458: If someone posts a channel link to channel_A that you don't belong to, it doesn't render properly (#7833)

* add channel link hints to post props

* optimization

* update regex, add unit test

* fix rebase issue

model/post.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	"regexp"
 	"strings"
 	"unicode/utf8"
 )
@@ -294,6 +295,22 @@ 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 (r *PostActionIntegrationRequest) ToJson() string {
 	b, err := json.Marshal(r)
 	if err != nil {