Allow "Sign with MFA" to be applied as a comment action without requiring "CAN_EDIT"

Summary:
Fixes T13307. We currently require "CAN_EDIT" to sign actions, but it's fine to sign a comment with only "CAN_INTERACT".

Since the actions like "Accept Revision" already work like this, the fix is one line.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13307

Differential Revision: https://secure.phabricator.com/D20574