add 'in list' and 'empty' filters, allow changing to filters with options, add js versions, fix bootstrap3+select

This commit adds three new filters:
* empty - shows NULL rows
* in list - uses select2's tag cloud, allows filtering multiple items from the same column using an "OR" condition
* not in list

It also adds versioning to the filenames of the javascript inside of admin/js. This prevents issues with caching.

The bootstrap3 select2 boxes also required fixing, so I included this CSS: https://fk.github.io/select2-bootstrap-css/