Add extra_data support to all comment types.
FileAttachmentComment had an extra_data field and API support for

setting it, which was handy, but this can be handy for other comment

types as well.
This support now exists in BaseComment, and all comment REST API support

has been updated to allow getting and setting it.
While I was at it, I moved some of the common field definitions into

BaseCommentResource, instead of duplicating it in the other base comment

subclasses.

All unit tests, new ones including, pass.