Add HTML header for Atom/RSS discovery on Phame blog pages
Allow easier discovery and subscribing to Atom feeds of Phame blogs.
To be extra cautious, also make sure that the added line is a PhutilSafeHTML.
Original author: @20after4
Test Plan: Go to /phame/blog/view/1/ and check the HTML source code. See an additional <link> item with type="application/atom+xml" in the <head> section.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, speck, tobiaswiese, valerio.bozzolan, Matthew, 20after4, Cigaryno
Maniphest Tasks: T15550
Differential Revision: https://we.phorge.it/D25346