Clean up supplemental capabilitiy checks in transaction edits

Summary:
We have this commented-out chunk of code now which was originally buggy and is now just nonfunctional.

For now, the core edit types don't always require CAN_EDIT (e.g., subscribe, comment, add edges), except for editing the edit policy itself, which always does. Add a supplemental capability check there and let everything else go through with CAN_VIEW. We can buff the policy checks on application editors over time, they all require appropriate capabilities to get to in the first place anyway.

Test Plan: Created and edited some tasks without getting overwhelmed with policy exceptions.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7351