Partially picked: DRY: added template tag to reference static page URLs

Pick note: Picked the code for the new template tag (because we might need
    it in the future) but don't pick the changes in the templates because
    we don't need to use it right now.

Conflicts:
	pootle/templates/editor/units/edit.html
	pootle/templates/layout.html
Partially picked: issue evernote/pootle#61

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

Conflicts:
	pootle/apps/pootle_app/management/commands/refresh_stats.py
	pootle/apps/pootle_store/models.py
Cleanup: coding style fixes

Conflicts:
	pootle/apps/pootle_store/models.py
Hide last updates rows if there is nothing to show. Fix evernote/pootle#61.
Show "State: unit created" as a first record in the timeline. Fix evernote/pootle#60
Cleanup: In the UI we use string, not unit
Partial pick: Show the number of units, not the total number of checks (fix evernote/pootle#66)

Pick notes: Didn't pick the changes in refresh_stats management command.
    Also had to revert some changes we did in 2acc09db to be able to
    apply some of the changes, and doing that we lost the ability to
    get check counts by category.

Conflicts:
	pootle/apps/pootle_app/management/commands/refresh_stats.py
	pootle/apps/pootle_store/models.py
	pootle/apps/pootle_store/views.py
	pootle/core/mixins/treeitem.py
Always show "Show timeline" which is not empty because of evernote/pootle#60
Fixed body scrolling issues

This issue existed since we introduced the sidebar in 4ea258.

When the content's height is low and doesn't cause any scrolling, it's
necessary to dynamically calculate the sidebar's height using JavaScript
(sigh) in order to stretch it down to the footer.
Be explicit when doing comparisons
Cleanup: move import to top, initialize dictionary with value

Conflicts:
	pootle/apps/pootle_store/views.py
Align styling
Cleanup: Remove unused imports

It is easier do this by ourselves than pick the Evernote commit and fix
the conflicts.