Hide "Signed with MFA" stories from feed

Summary:
See https://discourse.phabricator-community.org/t/sign-with-mfa-action-on-differential-revisions-creates-strange-feed-entries/2346.

If you "Sign with MFA" and only add a comment, we can produce a weird notification and feed story. Hide these stories from feed since they're broadly not interesting.

Test Plan:

• Used "sign with MFA" and only posted a comment.
• Observed feed.
  • Before: Janky "untitled story".
  • After: Sensible "added a comment" story.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20071