Make archive images uploadable through admin

Means you can re-upload or upload new images entirely from the admin.
Run archive tests on thumbnail to speed up tests
Backport ignored from python 3.4
Make it possible to only bootstrap a subset of apps
Exclude dev-only features from coverage
Remove WITH_COVERAGE envvar

Rather run the coverage tool directly on travis, this makes sure that
imports are counted, which it won't be if run through django-nose.