Admin → Users: accept an invited user's invitation for them, with a password the administrator sets #653

An invited local account (#322) signs in only once its user has followed the invitation link and chosen a password. An administrator who sets an account up for someone and wants to hand it over ready to use (an install without email, a user without a mailbox, an account set up by hand) cannot finish the invitation: the account stays invited or invitation expired.

Expected

• POST /admin/users/{id}/invitation/accept with {"password": "…"} accepts the invitation on the user's behalf: the password becomes the account's, the account is no longer invited (status active), and its invitation, if there is one, is marked accepted, so its link stops working. One transaction.
• It works whether a live invitation waits or not (invited and invitation_expired).
• The password follows the invitation page's rules (12 characters to 1024 bytes); 400 otherwise.
• 409 for an account that is not an active invited one (its user chose a password already, an account of another realm, a service account, a deactivated account); 404 for no account.
• Admin → Users → <user> offers Accept invitation for an invited account, asking for the password twice.
• docs/deployment.md (Inviting users) describes it: an administrator sets another person's password only this way.

Acceptance criteria

• Server tests: accepting sets the password (the user signs in with it), clears the status, makes the old link answer as invalid; 400, 404 and 409 as above.
• A web UI test for the action.
• A changelog fragment feature.