Update PassphraseCredential to support Mentions

Summary:
This revision allows us to see where passphrase credentials are mentioned in other parts of phabricator

This is very useful to "What is this credential for?" If we can see where it was been mentioned

Test Plan: Mentioned a passphrase and saw it in the transaction list

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

Subscribers: Matthew, Cigaryno, speck, tobiaswiese, valerio.bozzolan

Differential Revision: https://we.phorge.it/D25032