This ports the file attachment and draft file attachment models to TypeScript

and spina. We enable extra data support on the models, which we don't use

for any particular reason right now but might need to use in the future. This

also ports the draft resource child model mixin to TypeScript/spina.

Tested uploading new file attachments.
Tested dragging and dropping to upload new file attachments.
Tested viewing file attachments.
Saw that published file attachments appear correctly.
Tested dragging and dropping an image file into a review reply.
Tested deleting file attachments.
Tested updating file attachments.
Tested posting review request drafts with file attachments.
Ran JS unit tests.