in delete button tests: make sure teacher is part of facility
use an updated version of firefox for travis
Adds wait for alert_is_present on generic method for accepting
modal dialogs.

Conflicts:
	kalite/testing/browser.py
simplify test_delete_facility test
simplify test_delete_group test
metadata for 0.12.6
Merge branch 'release-0.12.0' into feature/2237-Auto-generate-screenshots-for-user-manual