Make sure group permissions are not overridden when not needed

Problem/Motivation

In the last release we made sure that the Group Flex logic in the Group form is always saved. This works well when the group visibility or joining methods are flexible. It overrides the permissions for the Group on a group level. But this (a) causes an issue when the group joining methods is not set. Since the "Join group" will be removed for Public groups with this joining method and (b) might create more issues in the future when maintaining multiple group types when there is existing Groups.

Proposed resolution

When only one or none of them can be changed we need to make sure the permissions are not overridden. Before calling the group-level save methods let's make sure to check if we actually need to save it.