New resource for user file attachments.

New file attachment resource was creating using the frontend backbone/RB.BaseResource model and post/put/get were tested manually.

Successfully created a file attachment user content is always empty upon creation as expected
Successfully posted caption and file data
Successfully modified and saved caption data
Tried to update file a second time and observed the expected behaviour (it failed)
Manually tested get_list by making a request to /api/file-attachments/, it returned a list of all file attachments owned by the user making the request as expected.