Partial pick: Fix bux introduced in [62eabe8], add invalidating cache for last_updated flag

Pick notes: The change in TreeItem was already picked. The change in
    flag_for_deletion doesn't apply since we removed that code. The
    remaining change was picked.

Conflicts:
	pootle/apps/pootle_store/models.py
Cleanup: removed some vendor-prefixed properties

The unprefixed versions can be used in the browsers we support.

Conflicts:
	pootle/static/css/login.css
	pootle/static/css/style.css
Cleanup: removed IE-specific stylesheet

Conditional comments are deprecated since IE 10.
Followup: Remove additional vendor-prefixed properties
Partial pick: Calculate Unix epoch using Django's own dateformat module

Pick note: Didn't pick the changes in refresh_stats management command.

Conflicts:
	pootle/apps/pootle_app/management/commands/refresh_stats.py
	pootle/apps/pootle_store/models.py
Use a `dateformat` template filter to display localized dates

The new `dateformat` filter allows extra formatting options and it can be
used in other scenarios too.

Conflicts:
	pootle/apps/pootle_store/views.py
Simplify code, be explicit