Switch to using allowed_mimetypes in webapi resources.
Review Request #4809 — Created Oct. 20, 2013 and submitted
Switch to using allowed_mimetypes in webapi resources.
The allowed_item_mimetypes and allowed_list_mimetypes were deprecated in
Djblets 0.6, but a few of our resources still use them. They'll be nuked
from Djblets soon, so I've updated our code here to no longer use them.
Unit tests pass.
