For as long as Review Board has existed, we've disallowed commenting on

items before they're published. The reasoning here was that if you add a

diff comment, replacing the diff will delete the comment (and similar

for file attachments).
This change updates the comment dialog to remove the hasDraft checks.

This is replaced with a new mechanism whereby we add a public state to

diffs and file attachments, and use that to show a warning to the user

if they're commenting on something which has not yet been published. The

bulk of the code changes here are just plumbing that state all the way

down to the comment block models, which use it to decide what to display

to the user.

Ran unit tests.
Created review requests with published and unpublished diffs and file

  attachments. Checked that adding a comment to published items worked

  as normal, and that the draft warnings were correctly shown for draft

  diffs and file attachments, as well as for interdiffs and file

  attachment diffs involving an unpublished tip revision.