Partial pick: Cleanup: removed dead code

Pick notes: Removed `group_by_count` and `group_by_count_extra`, but
    kept the rest of the code removed in the original commit since we
    still might need it in the future.

Conflicts:
	pootle/apps/pootle_misc/aggregate.py
Hide the ‘Quality checks’ section in the extended stats pane
Don't advance to the next unit if there are critical checks pending

Fixes evernote/pootle#53.

Pick notes: Had to adjust the template to keep our link to checks
    explanation.

Conflicts:
	pootle/apps/pootle_store/views.py
	pootle/templates/editor/units/edit.html
Change editor background color when the unit contains errors, blink error checks on submission in a different way
Reintroduce the checks HTML update that was forgotten in the last commit of mine
Cleaner look and better layout/wrapping of failing check indicators
Render disabled editor textarea in a more subtle way
Set the border width to 1px (this will look better on Firefox)