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.
Ah, yeah. Where would you prefer? I could stick it in `amo/` but that'll eventually get moved to `mkt/` anyway.