Transactions - hide "mentioned in X" story if you can't see X

Summary: ...also re-jiggers all the anchor stuff to use $xaction ID. This seemed like the simplest way once I got in the code, as well as having nice properties for if / when we want to re-add some ajax stuff since the ID is a pretty solid piece of data to key off. Fixes T6083.

Test Plan: mentioned DX in private DX+1. Could see on DX the mention as me and not as the other user. For transactions, I left a comment on Paste and it worked, and I edited an existing transaction and it worked.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T6083

Differential Revision: https://secure.phabricator.com/D10488