MM-15485: In-channel system behaviour for at-mentions and slash commands for group-constrained channels. (#10855)

* MM-15108: Prevent non-group-members from being added to group-constrained channels.

* MM-15485: Updates ephemeral message when non-group member is invited to group-constrained channel.

* MM-15485: Prevent group-permitted members from being removed from group-constrained channels. Show custom ephemeral when attempting /kick or /remove.

app/notification.go

@@ -135,7 +135,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 				var outOfChannelMentions model.UserSlice
 				var outOfGroupsMentions model.UserSlice
 
-				if channel.GroupConstrained != nil && *channel.GroupConstrained {
+				if channel.IsGroupConstrained() {
 					nonMemberIDs, err := a.FilterNonGroupChannelMembers(channelMentions.IDs(), channel)
 					if err != nil {
 						return nil, err