33: Admin Settings page

In order to get user-friendly way of configuring the app than using occ, we need to implements a Admin Settings page.
No idea if this should be in its own Admin Settings tab or merged to Sharing (or somewhere else...)

Must have entries:

• Enable Federated Teams and used frontal domain. (option is disabled by default and the entry for user domain is empty, format is https://cloud.example.net/)

Could be nice in the future:

• Hard moderation, when switching a member level (value can be 0, 1, 2):
  • 0: moderator can be same level than current AND final assigned level (moderator can promote same level, demote from same level)
  • 1: moderator can be same level than final assigned level but MUST be above target current level (moderator can promote same level, cannot demote from same level)
  • 2: moderator must be above current AND final assigned level, (moderator cannot promote to same level and demote from same level)
• Members limit: limit member counts in a single team (default: -1)