Also update TP counts on unit deletion
Save the counts after unit deletion
Also update TP counts

The calls for saving the Store and the TP have been moved to the bottom
of the function.
Add fields to store the number of units with failing critical checks
Rename TreeItem method

In the future we might want to expose counts for regular checks as well.
Retrieve critical counts directly from the model fields

This way we avoid expensive calculations and we align with the
word counts and suggestions counts way of retrieving the data for
the stats.

This partially undoes 8483d758 so get_checks() returns the data
for all kind of checks.
Calculate critical counts in refresh_stats
Update critical counts on unit deletion
Update critical counts on unit saving
Stats: Don't cache counts

This eases testing. Also since we seem not to be clearing the cache we
can't safely rely on the cache.
Update critical counts when muting/unmuting critical checks
Drop TreeItem's refresh_stats

Evernote uses it in refresh_stats management command, but we changed it
to refresh count fields on DB instead of calculating everything and
save the results in the cache engine.
We never pass a submission to _get_last_action
Simplify _get_last_action

There is no need to perform several queries.
More performant way to retrieve last action
Settings: Adjust suggested cache settings

Provide CACHES for dealing with the suggested way (memcached) while
commenting it to fallback to default caching.
Settings: Use local-memory caching by default

For toy deployments it is fast enough.
docs: update Wikipedia page when releasing
Simplify context processor
Stats: Display stats table headers in one line

This removes the ugly stretched columns and aligns us with Evernote.
The bastard was hard to find.