Refactor delete_cache_for_path in Goal model

Now uses pootle_path instead of a TP and a Store.
Flush goal stats caches when stores change

The cache for the goal stats is now flushed when:

* A file is uploaded
* A new translation is sent
* A translation is blanked
* A translation is marked as 'needs work'
* A suggestion is accepted
* An update against templates is performed
* An update to VCS is performed

Partially fixes bug 3130