Add initial skeleton outline of backdrop_users (or dataset_users?)

- Adds a many2many relation with data_sets and an email field
- Also adds very basic view of users in the admin app, but that doesn't work yet
Working admin and test for unique email
wowee zowee it's a check_disabled decorator!
email not name
Add serialization to backdrop user model

- Roughly copies the serialize method on data_set model
Add the fixtures we were actually using back into data_set view test
Add url route for user lookup

- new matcher based on looking up by email in url
- implies a split in views in the import
Add in test data and view tests for backdrop user views
Stricter ignoring of log files in .gitignore
Remove debug printing
Add in common authorization methods to backdrop user view tests

- TODO refactor this into a shared/auth test, as the methods are shared across views
pep8 the email url matcher
Remove the atomic aspect - no need for this transaction
 LOL 
Adding in test data for backdrop user import management command
Factor out common auth and cache logic
Add user:dataset count to backdrop user admin

- Sortable by number of data-sets the user has access to
- Vaguely useful, but mainly an exercise in seeing how easily we can customise the output of the manytomany field (not very easily)
Tests for purge on backdrop user save and delete
working backdrop user purge on save and delete
Reduce the number of data-sets

- Drastically reduces the amount of hand coded fixutre information we'll need to write to test dataset lookups in the backdrop user import script tests