Moved apps/addons/ to mkt/webapps/ (bug 1021189)

Note: I'm leaving addons/fixtures/ for now since other tests under apps/ may use them. Once we're done moving apps we theoretically can delete them all b/c we've updated the tests.
I thought as long as we haven't merged `Addon` and `Webapp` yet it was still functionality we expect to work, so I left the tests.