Don't post Out Of Office emails as PR comments (BB-11385)

Problem:

• PR notification hit my inbox
• Out Of Office email was sent
• The raw HTML of the OOO email was posted as a comment on the PR

It'd be good to filter these out. Also, posting the encoded HTML probably wasn't the intention in any case

Screenshot attached. It was sent by MS Exchange email server, if that helps. (Yeah I know, blech, but it's what we have)