Add check field to Submission model

Pick notes: Style adjustments in field declaration, and recreated the
    migration from scratch to avoid surprises.

Conflicts:
	pootle/apps/pootle_statistics/models.py
Partial pick: Enable muting and unmuting of quality checks, track such
actions as last actions (evernote/pootle#56 evernote/pootle#54)

Pick notes: Picked most of the changes. Moved changes in `pootle.core.log`
    to `pootle_store.util`. Replaced `pootle-staticpages-display` URL
    with hardcoded URL to our checks docs. Didn’t pick the change in
    `flag_store_before_going_away`. Also did minor tweaks in the templates.

Conflicts:
	pootle/apps/pootle_store/models.py
	pootle/apps/pootle_store/urls.py
	pootle/core/log.py
	pootle/templates/editor/units/xhr_checks.html
Minor refactoring

Pick notes: Also moved our language picker code to the common.js init.

Conflicts:
	pootle/static/js/common.js
Moved dropdown's fixing code to the browser script

Also includes a minor refactoring.
Refactored the way zoom handlers are set
Updated docstring

Conflicts:
	pootle/core/browser.py
Partial pick: Removed dead code

Pick note: The other changes were already picked in the past.

Conflicts:
	pootle/apps/pootle_project/views.py
	pootle/apps/pootle_translationproject/views.py
	pootle/core/browser.py
Incorporate actual project object in the context

Conflicts:
	pootle/apps/pootle_project/views.py
Simplified naming

Pick note: Also altered all the other references to `get_resource_context`.

Conflicts:
	pootle/apps/pootle_store/views.py
	pootle/apps/pootle_translationproject/views.py