refresh_stats: Add some additional prints to report progress
refresh_stats: Marginal performance improvement on large projects
Stats: Add fields to cache last created unit and last submission

This will improve performance when retrieving the stats table.
Stats: Update last created unit

Current approach will increase load when adding whole new stores,
translation projects and projects.
Stats: Update last submission
refresh_stats: Update last unit and last submission fields
Stats: Update and retrieve cached unit and submission
Minor cleanups
Account URLs: Use styleguide conventions
Bugfix: Use Django ORM exclude properly
Roll back transactions in error pages
Cleanup: moved navbar-related CSS into a separate file
Navbar: switch to `nav` tag
Cleanup: use `background-color` instead of `background`
Navbar: also highlight on focus
Navbar: added basic responsive styles
Navbar: simplified layout
Navbar: minor RTL fix
Navbar: Fix the language picker to better fit Evernote navbar
Select2: Properly load the spinner