Fix a minor text alignment issue for static text comment actions like "Accept Revision"

Summary:
Ref T13187. See PHI836. The "action" comment actions in Differential (Accept, Reject, etc) render a single line of descriptive text. This is currently slightly misaligned.

Give it similar sizing information to the label element to the left, so it lines up properly.

Test Plan:
Note that "Request Review" and "This revision will be..." are now aligned:

Reviewers: amckinley

Maniphest Tasks: T13187

Differential Revision: https://secure.phabricator.com/D19600