Our current UI allows users to comment on deleted file attachments, but these

comments cannot actually be saved for the deleted files. It also doesn't make

sense to allow commenting on deleted files. To address this, this change blocks

commenting on deleted files. When attempting to comment on one, we display a

warning message in the comment dialog and block the ability to edit the file,

similar to what we display when unauthenticated users try to create comments.

We update the FileAttachmentCommentBlock and FileAttachmentReviewable

models to make use of the file attachment states concept that were recently

introduced.
This also hides the Markdown docs link for comment dialogs that cannot be

edited. This makes more sense than hiding it only for unauthenticated

users, which is what we were doing before.

Ran unit tests and JS tests.
Tested commenting on a deleted file, saw the warning and inability to

  edit/save the comment.
Tested commenting on a non-deleted file.
Tested commenting on a draft file.
Tested the above cases while logged in and while not logged in.