When there are more than a 1000 items in the table, retrieving and
displaying all items takes about 7 seconds.
This is every time we refresh the items page, which is too slow.

Having the description field only on the Model class makes things
inconsistent.
It's nice to have a description on other attributes as well.

The generation of the QR code image is quite slow and makes the page
displaying them take a long time to load when it includes more a few.
The data never changes and can easily be cached (the repr() of the class
is used in the cache key, so if the name or id changes, the cache will
be recomputed - there is no need for a timeout).

Column not visible but searchable.
Allow to search comments and TAG number for items imported from JIRA.

text is really generic and could conflict with another variable.
body is often used for comments.

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

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.

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.

TODO: we should be able to render HTML in the comments
Look at https://github.com/miguelgrinberg/Flask-PageDown

- Main blueprint used to declare custom views for all blueprints.
(keep inventory blueprint cleaner)
- Add url_prefix to each blueprint

Store some settings to the session to make it easy to register
several identical items.

main blueprint splitted in:
- inventory
- networks

Use the session instead of URL parameters to store the last network
chosen.

- refactor Network class

Catch exception when no network exists (null is passed as network_id).

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.

Most items won't have a name.
hostname could be added in a separate table if required.