More control over invites and public sharing

[Neriderc]

From what I can tell there is currently only one type of user account (i.e. there's no admin type account).
The features to invite others and create public links are great! But I'd like control over them.
For example, maybe only an admin user can use them.
Maybe an admin user could enable or disable them by group.
Maybe there could be a setting where people could be invited but need to be approved by an admin once they create their account.
Maybe there could be single use invite codes that can only be used once (and optionally only an admin can create these).

I recognise that this could be quite a lot of work, so in the mean time there could just be config values to enable/disable inviting or the creation of a public link for posts.

[qwertfisch]

Also the admin should be prevented to leave the group, except there is another admin present.

If the group does not have or use admins, the last user should not be able to leave the group (or should be asked to confirm the choice). I managed to remove the demo user from the demo group without confirmation.

[nrobinaubertin]

If we don't allow the last user to leave the group, then we need a way to delete a group. In the current situation, the only way to delete a group is that every user leaves it (and the group gets garbage collected by a cron job later).