Admin approval of new users

[abretaud]

Describe the bug

It could be useful to add an optional step of admin approval for newly created user accounts. Some security authorities are not very fond to have completely open registration and could force to have this kind of validation step to avoid (some) bot/hostile accounts.

Additional context
Add any other context about the problem here.

[hexylena]

mastodon has a nice implementation, enables admins to require users to "write a brief bio/description of why you're applying" when they apply.

[abretaud]

ah yes, we ended up adding it to our non-galaxy account management app, very useful too!
(of course we sometimes get "to do science", but not so often)