site.py: check if site is enabled for uploads

Fix Bug 69090: upload.py attempts uploads on wikis that
don't allow uploads.

Check APIError code to determine if Site is enabled for uploading.
Currently it is done and cached at first upload attempt.

Added also a helper function is_uploaddisabled() to determine if a site
has uploads enabled, based on a dummy upload attempt.
Throttle has been disabled for such request to speed up the query
(implementation taken from I80b2bba9e63832173d5b697db1f4ea419ca1122f).

Change-Id: Id8ae0cd51582d76b7bdd15c53e39b4105995b540
Merge "site.py: check if site is enabled for uploads"