backend: rework sign-up checks to enable "overwrite when auth"
backend: rework sign-up checks to enable "overwrite when auth"

* Reworks sign-up checks so that user that are authenticated can do
  whatever they want and only get a warning instead of an error.

* Now emits all errors and not only the first one.

Signed-off-by: Marco Neumann <3829486b93ec44395f0b980424bae9b6fb07b7bc@crepererum.net>

spz-signup/spz-signup