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