Added an Item-Content-Type header to Web API responses for list resources. This header provides the mime-type for the item resource of the list.

Introduces a new 'allowed_mimetypes' attribute to WebAPIResources, and starts the process of deprecating 'allowed_list_mimetypes', and 'allowed_item_mimetypes'. Any items missing from 'allowed_mimetypes' found in 'allowed_list_mimetypes', or 'allowed_item_mimetypes' will be appended during initialization to be backwards compatible.

Wrote new test cases to check the generated Item-Content-Type header, ran all Djblets tests, and manually tested in a web browser.