Adding support for dragging and dropping images into the markdown editor. Instead of having upload their image first and then link to that image, users can simply drag-and-drop their image file into the markdown editor. This will upload the image to reviewboard and insert a link to that image in the editor.
These changes focus on the backend work involved. This includes:

- Extending FileAttachment Model to include a User, Local Site and UUID field

- Creating a view that will redirect to a FileAttachments' file given the FileAttachment's UUID. This view can be accessed at /users/<username>/file-attachments/<uuid>/

- Creating a Web API for user file attachments, UserFileAttachment resource.

- UserFileAttachment resource can be accessed at /api/users/<username>/file-attachments/<id>/

- Users can only get/modify/delete their own UserFileAttachments. Superusers can get/modify any.

- UserFileAttachments can be created (using POST) without an associated file, in this case the database entry will be created, and the view will return a 404.

- UserFileAttachments can be modified (using PUT) to include a file. This can only be done to UserFileAttachments that do not already have an asosciated file.

- When deleting a UserFileAttachment, the associated file is also deleted.
Also did some refactoring of the base_file_attachment Web API resource so that it could be extended by both user_file_attachment and review_request_file_attachment resources.
See https://reviews.reviewboard.org/r/6220/ for initial prototype.

All unit tests pass after making the changes.
Extensive manual testing of UserFileAttachment Web API resource using Postman REST client:

- Testing each method (GET/POST/PUT/DELETE) for UserFileAttachment as a regular user and superuser.

- Testing local_site functionality
Added unit tests around the changes to the FileAttachment model:

- Tests around creating a FileAttachment with/without a file

- Tests around the is_accessible_by(user) and is_mutable_by(user) methods
Added unit tests around the new UserFileAttachment WebAPI Resource.