It shouldn't be possible to change the ICS id of an item.
But when registering a bunch of existing items via the API, we need to
allocate an ics_id otherwise not much can be done in the interface.
The id is required for the view and edit page.

The solution is to define some "temporary" ids that don't correspond to
any pre-printed labels.
Items with a temporary id can be assigned a real ics_id when assigning a
label.

Reserved ids start with 'ZZ' (from ZZA000 to ZZZ999)

ICS id can now be non nullable.

Allow to customize the default selected values:
- the default network prefix can be defined in the configuration (default to 24)
- the default first and last IP chosen can probably be improved
  it defaults to hosts[9] and hosts[-5] if the list has more than 15
  elements or the first and last otherwise

Note that when using a small prefix (< 19), the number of possilbe IPs is quite
large and updating the first and last IP select field takes some time...

In Flask, session data are stored in a cookie by default.
Using a server side session, there is no risk of putting too much data
in the session and transmissting a big cookie on every request
(only an id is sent in the cookie).

Redis is used as backend (without persistent storage).

- All created tokens are stored in the database.
  To revoke a token, we just delete it from the database.
  Tokens not found in the database are thus considered unvalid / revoked.

- Add button to copy generated token to the clipboard

- Redesign profile page

This allows a client to login only once a day.

We might want to decrease that and look into refresh token.
But this looks overkill for our use case.