Use the app's default_locale for post-review notification mails (bug 848844)

Note: the mails themselves are currently sent with no_translation(),
and the templates don't have have translations anyways. This is just
for the app details such as the name.
Merge pull request #784 from diox/reviewer_notification_translations

Use the app's default_locale for post-review notification mails (bug 848844)
Pass signal to ModelBase's update
Moved over old search test to API
Cleaned up reviewer approve signals (bug 879047)
Added @write to indexing tasks (bug 879047)
Also index to new index upon review (bug 879047)
Added feature profile filtering to featured apps api (bug 875009)
add in a payment and upsell api (bug 878862)
remove until the replacement is ready (bug 878350)
Remove use of .decode() in feature listings (bug 877364)
remove region handling for now (bug 878118)