MM-14757/14758: Restricts deleting team and channel members if the team or channel is group constrained. (#10553)

* MM-14757/14758: Update APIs to reject removals from group-constrained teams.

* MM-14757/14758: Tests API changes.

* MM-14757/14758: Allow users to leave channals and teams.

* MM-14757/14758: Updates translation key order.

* MM-14757/14758: Adds user to team before setting it to group-constrained b/c of new add restrictions.

api4/channel.go

@@ -1218,6 +1218,11 @@ func removeChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if channel.GroupConstrained != nil && *channel.GroupConstrained && (c.Params.UserId != c.App.Session.UserId) {
+		c.Err = model.NewAppError("removeChannelMember", "api.channel.remove_member.group_constrained.app_error", nil, "", http.StatusBadRequest)
+		return
+	}
+
 	if c.Params.UserId != c.App.Session.UserId {
 		if channel.Type == model.CHANNEL_OPEN && !c.App.SessionHasPermissionToChannel(c.App.Session, channel.Id, model.PERMISSION_MANAGE_PUBLIC_CHANNEL_MEMBERS) {
 			c.SetPermissionError(model.PERMISSION_MANAGE_PUBLIC_CHANNEL_MEMBERS)