reviewed convoytags.py

Yo @tedtieken 

This is built on top of `add-tests`, so its still an ungodly amount of files.  It'll make the most sense either: after `add-tests` is merged, or by looking at this diff view: https://github.com/tedtieken/django-asset-convoy/compare/add-tests...peter-review-pass

The two main things I did were:
* cleaning up logic in `convoytags` if statements
* aggregated settings into `convoy.settings`