The API resource implementation previously prevented POST requests to

item resources, and with a new change to Djblets, will also prevent PUT

requests to list resources. However, neither of these was properly

tested in the test suite.
This change updates our API test mixins to test these scenarios where

appropriate. Most of the individual test suites needed to add some setup

methods to handle this appropriately.
Unfortunately, the naming of the setup methods isn't ideal.

setup_http_not_allowed_item_test should really be

setup_http_not_allowed_put_test, and

setup_http_not_allowed_list_test should be

setup_http_not_allowed_post_test. These also do not currently support

similar test cases for when there is a local site (though that shouldn't

be a problem). Rather than trying to rename these methods now, I've just

gone with the old ones, and we can address the naming later. For this

reason I'm also holding off on adding missing docstrings or typing,

merely trying to match it to the individual style of each existing test

case.

Ran unit tests.