Make message and AccessControlledMessage public so they can be used by other plugins

[halkeye]

Does changing access levels need tests?

Looking to add pubsub support to favorites, and while I was doing so, I couldn't figure out how to make my own message type, especially one that didn't broadcast all fav changes that all users did to the entire system (should it do so?)

Submitter checklist

• [N/A] Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

[halkeye]

@olamy @stephenc @tfennelly