Add RSS/Atom feed support for user actions (#16002)

Return rss/atom feed for user based on rss url suffix or Content-Type header.

vendor/github.com/gorilla/feeds/AUTHORS

@@ -0,0 +1,29 @@
+# This is the official list of gorilla/feeds authors for copyright purposes.
+# Please keep the list sorted.
+
+Dmitry Chestnykh <dmitry@codingrobots.com>
+Eddie Scholtz <eascholtz@gmail.com>
+Gabriel Simmer <bladesimmer@gmail.com>
+Google LLC (https://opensource.google.com/)
+honky <honky@defendtheplanet.net>
+James Gregory <james@jagregory.com>
+Jason Hall <imjasonh@gmail.com>
+Jason Moiron <jmoiron@jmoiron.net>
+Kamil Kisiel <kamil@kamilkisiel.net>
+Kevin Stock <kevinstock@tantalic.com>
+Markus Zimmermann <markus.zimmermann@nethead.at>
+Matt Silverlock <matt@eatsleeprepeat.net>
+Matthew Dawson <matthew@mjdsystems.ca>
+Milan Aleksic <milanaleksic@gmail.com>
+Milan Aleksić <milanaleksic@gmail.com>
+nlimpid <jshuangzl@gmail.com>
+Paul Petring <paul@defendtheplanet.net>
+Sean Enck <enckse@users.noreply.github.com>
+Sue Spence <virtuallysue@gmail.com>
+Supermighty <ukiah@faction.com>
+Toru Fukui <fukuimone@gmail.com>
+Vabd <vabd@anon.acme>
+Volker <lists.volker@gmail.com>
+ZhiFeng Hu <hufeng1987@gmail.com>
+weberc2 <weberc2@gmail.com>
+