Use endpoint argument to avoid collision with flask-admin Network

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

main blueprint splitted in:
- inventory
- networks

See https://clipboardjs.com

Open source icon set with ready-to-use fonts for bootstrap
See https://useiconic.com/open/

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.

Clean code at te same time.
The macro from flask-bootstrap already takes care of everything.

- update flask-bootstrap
- update datatables to v1.10.15 for bootstrap 4 support