other notable changes:
* `secret` is now initialized with a 32 0-byte key
* `permissions` has a new colum called `hidden` which defaults to `false`
* new functions for key updating:
  * keys are generated by sha256(random data + previous key)
  * the random data is transmitted to door firmware,
    and after this was successfull, back to the database, which then
    also generates the new secret key
  * `gen_keyupdate`: create a message for the firmware that
                     invokes the key update
  * `update_signingkey`: call it after the door firmware key was updated
* `user_mod` function now selectively updates only changed user fields
* renamed `first_admin_enable` to `manual_admin_enable`
* remove `user_del` in favor of new `user_set_visibility`, which
  can hide/unhide a user from the user list. that way we can retain
  the change history and permission grant order
* the `setup.sql` file is now one transaction