Fixes INFRA-197

Pagination information (next, prev, first, last) is included in the Link
HTTP header.
The total number of entries is provided in the X-Total-Count HTTP
header.

Strings are automatically converted to datetime object.
Note that datetime are stored without timezone in the database (in UTC).

- This will help tracking who and what was created / changed
- Move fetch_current_user_id to utils.py
- Make ItemComment inherit from CreatedMixin

When registering a new host, only admin users shall be able to select
networks marked as "admin_only"

If a selectfield is disabled, None is returned (not a string).
To pass wtforms validation, that value shall be part of the choices.

We use None as the value for SelectField that are optional (instead of
'').
We were previously converting '' to None when creating a model.
We now use the coerce function to let the form convert 'None' to None.

Can use:

  -  /api/items?serial_number=123456
  -  /api/items?location_id=1

This can be done via the Admin interface but this latter shall be
restricted to admin users. This simple page allows the group "create"
to easily add new Manufacturer, Model or Location.

The serial number might not be unique and can't be used
as the string to hash.
There are for example SD cards that all have the same SN.

Using an ICS id allows to pre-print labels that can be assigned to
hardware when scanning serial numbers.
This id shall be easy to remember (unlike UUID).

The hash is supposed to be computed in the __init__ method of Item.
But flask-admin never passes any parameter when creating the model class.
The attributes are changed after creation.

When creating an item in the admin view, we use the on_model_change
method to update the hash.

There might be a better way to avoid this code duplication.
This works for now.