Allow Herald rules to add comments

Summary:
See PHI242. All use cases for this that I know of are pretty hacky, but they don't seem perilous, and it's easier than webhooks.

See P1895, T10183, and T9853 for me previously refusing to implement this since all those use cases were also pretty bad.

Test Plan:

• Wrote a rule to add comments, saw it add comments.
• Reviewed summary, re-edited rule, reviewed transcript to check that all the strings worked OK.
• Wrote a new rule for a non-commentable object (a blog) to make sure I wasn't offered the "Add a comment" action.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18823