Add field to permanently store failing checks count

We need this value too often, so using cache here is not optimal - it is
better to always have up to date value calculated.

This will also allow nice progress bars above translation level in future
(issue #196).
Update failing checks count on stats update
Use stored failing checks count while counting
Add data migration to fill in failing checks counters on migration
Add missing license boilerplates
Invalidate cache on checks deletion

When we delete checks for (now) a fuzzy message, the cache needs to be
invalidated.

This might be source of issue #233.
Invalidate checks cache on translation update

We might be more clever on this and do the cleanup conditionally, but
for now this is safe solution as the checks might become stale.

This completes fix for issue #233