Add Bugzilla.is_bug_confidential() method.

Clean up user include-field defaults.
Disallow posting to confidential bugs.

Remove [RB] prefix from attachment summary. Use x-review-board-request
MIME type to identify RB attachments. Include review request description
as comment when posting attachment.
Bump version.
Add model to map Review Board user ids to Bugzilla user ids.

Use Bugzilla ids when adding/updating users, since email can change.
Bump version.