Address a transaction issue with some audit actions not applying correctly

Summary:
See https://discourse.phabricator-community.org/t/cannot-accept-commits-in-audit/2166/.

In D19842, I changed PhabricatorEditField->shouldGenerateTransactionsFromComment().

• Previously, it bailed on getIsConduitOnly().
• After the patch, it bails on a missing getCommentActionLabel().

The old code was actually wrong, and it was previously possible to apply possibly-invalid actions in some cases (or, at least, sneak them through this layer: they would only actually apply if not validated properly).

In practice, it let a different bug through: we sometimes loaded commits without loading their audit authority, so testing whether the viewer could "Accept" the commit or not (or take some other actions like "Raise Concern") would always fail and throw an exception: "Trying to access data not attached to this object..."

Fixing the insufficiently-strict transaction generation code exposed the "authority not attached" bug, which caused some actions to fail to generate transactions.

This appeared in the UI as either an unhelpful error ("You can't post an empty comment") or an action with no effect. The unhelpful error was because we show that error if you aren't taking any other actions, and we wouldn't generate an "Accept" action because of the interaction of these bugs, so the code thought you were just posting an empty comment.

Test Plan: Without leaving comments, accepted and rejected commits. No more error messages, and actions took effect.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: stephan.senkbeil, hskiba

Differential Revision: https://secure.phabricator.com/D19845