Make join methods for groups re-usable

Problem/Motivation

Functionality for using different join methods is hardcoded only for groups.

Steps to reproduce

1. Define your entity type in the "social_group_join_method_usage" hook.
2. When you log in as CM+ and navigate to the entity creation page then you should see the "Join methods" field in the "Settings" section.
3. Join methods functionality for your entity type should work similarly to the same functionality for groups.
4. Join methods functionality for groups should work as before.

Proposed resolution

Delete hard relationships to groups to be able to use it for different entity types.