Unit: Use 'url' to identify the user url, rather than 'profile_url'
Bugfix in migration

It turns out that we can have suggestions without user. We can't merge
those, so just skip them.
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