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_test.go

@@ -6,6 +6,8 @@ package model
 import (
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestPostJson(t *testing.T) {
@@ -124,6 +126,11 @@ func TestPostIsSystemMessage(t *testing.T) {
 	}
 }
 
+func TestPostChannelMentions(t *testing.T) {
+	post := Post{Message: "~a ~b ~b ~c/~d."}
+	assert.Equal(t, []string{"a", "b", "c", "d"}, post.ChannelMentions())
+}
+
 func TestPostSanitizeProps(t *testing.T) {
 	post1 := &Post{
 		Message: "test",