Add option to toggle group requests

[linusgke]

Add a toggle-switch that allows group administrators to disable the necessity of accepting new users.

[lchristmann]

Two things are to be viewed separately here:

• if you JUST REGISTERED to the WaldorfConnect platform, you NEED TO BE ACCEPTED by a group admin (if you selected any groups to join when registering | otherwise another admin) ANYWAYS - doesn't matter if this group is open for anyone or needs accepting
• if you've ALREADY BEEN ACCEPTED TO THE PLATFORM after registration, you can join such groups that have accepting new users disabled immediately in "self-service"

The can be implemented as such:

• the first case is covered by a user status (ACCEPTED TO THE PLATFORM: accepted column in users table)
• the second case is covered by the users status in group memberships (ACCEPTED TO A GROUP: accepted column in group_membership table)

Now if a user is in the first state, he always needs to be accepted, while in the second case, he can be allowed to immediately join groups that allow this.

This leads to the following rules:

• If a freshly registered user is accepted to a group, his status is accepted to BOTH THE PLATFORM and THIS GROUP.
• If a freshly registrered user is accepted by an admin via the User Administration and not to a group, only his accepted TO THE PLATFORM status is set.
• An "old/established" user has to be accepted to the groups that require it, but can join groups all by himself that have accepting new users disabled.