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/command_invite.go

@@ -142,8 +142,14 @@ func (me *InviteProvider) DoCommand(a *App, args *model.CommandArgs, message str
 	}
 
 	if _, err := a.AddChannelMember(userProfile.Id, channelToJoin, args.Session.UserId, ""); err != nil {
+		var text string
+		if err.Id == "api.channel.add_members.user_denied" {
+			text = args.T("api.command_invite.group_constrained_user_denied")
+		} else {
+			text = args.T("api.command_invite.fail.app_error")
+		}
 		return &model.CommandResponse{
-			Text:         args.T("api.command_invite.fail.app_error"),
+			Text:         text,
 			ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL,
 		}
 	}