Add /api/browsers?slug=<slug> filtering

Update docs to use filtering rather than alternate ID lookup
Refactor model creation in tests
Add filters to browser-versions

- browser (by ID)
- browser__slug
- version
- status
- or a combination of above
Explode tests/test_viewsets.py
Add filter by username to users
Add filters to historical-browser

By browser ID, slug
Add filter by id to historical-browser-version
Merge branch 'feature/filtering_13' into develop

Fixes #13