R1:612e9c6e09ad

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…