Replace deprecated AphrontView::setUser()/getUser() calls with setViewer()/getViewer()
Summary:
`AphrontView::setUser()` and `AphrontView::getUser()` are deprecated. See rPaaab1011. Save a few CPU cycles.
Followup to rP4d98c0a4.
Test Plan: Use an IDE to find calls to replace (and not interfere with other existing `setUser()`/`getUser()` methods).
Reviewers: O1 Blessed Committers, valerio.bozzolan, mainframe98
Reviewed By: O1 Blessed Committers, valerio.bozzolan, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26485