Count Github PR and Reviewboard attachments as patches too (issue 58)

Also count attachments with a mimetype of "text/x-github-pull-request"
or "text/x-review-board-request" as a patch, when determining whether a
bug should appear on "to checkin" and "to fix" - rather than treating
them like a normal non-code attachment.

Fixes https://github.com/harthur/bugzilla-todos/issues/58