Migrate away from Flask_script

[nicolasberens]

We have a few helper scripts that are run by manage.py using flask-script

usage: manage.py [-?]
                  {db,resetdb,remove_unconfirmed_requests,remove_orphaned_prefixes,import_legacy,get_mails_for_legacy,mail_addresses_for_pools,mail,shell,runserver}
                  ...

 positional arguments:
   {db,resetdb,remove_unconfirmed_requests,remove_orphaned_prefixes,import_legacy,get_mails_for_legacy,mail_addresses_for_pools,mail,shell,runserver}
     db                  Perform database migrations
     resetdb
     remove_unconfirmed_requests
     remove_orphaned_prefixes
     import_legacy
     get_mails_for_legacy
     mail_addresses_for_pools
     mail

flask-script is deprectaed and we should use the flask cli