Bugfix: Editing of instructions

The instructions were not properly shown after canceling or submiting
the editing form.
pep8 checker: Profiles app is now gone
missing parameter for Apache proxying

To be able to use Apache proxying, one needs to add a ProxyPreserveHost parameter. One also need to adjust the Pootle configuration file.
Drop django-maintenancemode

We have decided against it as it has not been updated in a while.
Users who wish to use maintenancemode can easily install and add it
in their settings.
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
Cleanup: moved navbar-related CSS into a separate file

Cherry-Picked-From: 0211f1578bfb564801e12da138166461c98dffce
Navbar: switch to `nav` tag

Cherry-Picked-From: ed27f0c93381ebae30d249f370b02d3fb9514837
Cleanup: use `background-color` instead of `background`
Navbar: also highlight on focus