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. Testing Done: Ran unit tests.