coverage: enable test coverage
Unbreak update_against_templates with goals

It was broken since b3edd98 as post_template_update and post_vc_update
signals no longer have ‘oldstats’ or ‘newstats’ args, so check for their
presence first.
Add missing import

pootle_store/decorators.py was missing Store import and thus caused
tracback with: NameError: global name 'Store' is not defined trying to
download a file.