Consolidate and rename some static API endpoints

These new names are clearer, and we do not need to differentiate between
pre-release and development branches, really.
Make setting inhibit flag part of init

This is slightly better, as we can set an inhibit status if an exception
is thrown during an update attempt.
Set inhibit on a thrown exception during update

This prevents a user from making a possibly-bad situation worse, if an
update goes wrong.
Implement updates

Does this via a subclass of the GetAddons class, which looks to work
pretty well.