Add 'oa_comment' message type for comments (will be used by oa_mailhandler)

Currently, only Work Tracker uses comments in Open Atrium. It does this by creating it's own specific message type. However, other plugins may use comments in the future! This includes ones that users make for their own use and don't distribute publicly.

So that every one doesn't need to create it's own message type (and so that oa_mailhandler will work with them all), oa_messages should provide it's a 'oa_comment' message type!

I'll attach a patch in a moment that does this. Most of the code was taken from oa_worktracker.