stack_member was incorrectly coerced to a string

JIRA INFRA-1648 #action In Progress

Validation worked in the models because fullmatch is used but in the
Regexp validators from wtforms, only match is called.

The form validator didn't catch "AAA1100".
Test added for this.

JIRA INFRA-1569 #action In Progress

JIRA INFRA-618 #action In Progress

https://black.readthedocs.io/en/stable/

This is to be used for stack of switches only

- a stack_member is linked to an host (device_type has to be "Switch")
- a stack_member is an integer between 0 and 9 (included) or can be None
- the couple (host_id, stack_member) must be unique

JIRA INFRA-267

1. When we have a stack of switches, the same IP is associated to several
   switches
2. Move the items <-> host assocation to the item create/edit page

JIRA INFRA-267

Allow to keep track of batches of small parts.

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

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

Move regex to validators.py

Used to validate a list of strings against a regular expresssion.

To check if the unique object is the one being edited, we need to store
the object used to create the form in the form (_obj).
This is inspired from flask-admin.

Rendering done client side using Showdown: http://showdownjs.com
Live preview is shown below the form.
Only raw text is saved in the database (no HTML).

Note that no XSS filtering is done!
See https://github.com/showdownjs/showdown/wiki/Markdown's-XSS-Vulnerability-(and-how-to-mitigate-it)

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 splitted in:
- inventory
- networks

- refactor Network class

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.