Fixed bug in settings.py
Make database paths configurable from settings.ini

This adds a new requirement: dj-database-url. Database URLs can be set
from either `settings.ini` or env variables.
Improved settings.ini documentation
Added row highlighting of selected session in inventory view
Added entity and entityrole model creation to import utility
Added Ajax paging to regid view
Replaced broken tags in db

- Replaced broken tags in db
- Added script to create some useful entries in db for testing purposes
Check for existing session in swid inventory view

Show an error message when viewing the inventory view for a device that
has no sessions.

Fixes tnc-ba/strongTNC#158.