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/doc.go

@@ -0,0 +1,73 @@
+/*
+Syndication (feed) generator library for golang.
+
+Installing
+
+	go get github.com/gorilla/feeds
+
+Feeds provides a simple, generic Feed interface with a generic Item object as well as RSS, Atom and JSON Feed specific RssFeed, AtomFeed and JSONFeed objects which allow access to all of each spec's defined elements.
+
+Examples
+
+Create a Feed and some Items in that feed using the generic interfaces:
+
+	import (
+		"time"
+		. "github.com/gorilla/feeds"
+	)
+
+	now = time.Now()
+
+	feed := &Feed{
+		Title:       "jmoiron.net blog",
+		Link:        &Link{Href: "http://jmoiron.net/blog"},
+		Description: "discussion about tech, footie, photos",
+		Author:      &Author{Name: "Jason Moiron", Email: "jmoiron@jmoiron.net"},
+		Created:     now,
+		Copyright:   "This work is copyright © Benjamin Button",
+	}
+
+	feed.Items = []*Item{
+		&Item{
+			Title:       "Limiting Concurrency in Go",
+			Link:        &Link{Href: "http://jmoiron.net/blog/limiting-concurrency-in-go/"},
+			Description: "A discussion on controlled parallelism in golang",
+			Author:      &Author{Name: "Jason Moiron", Email: "jmoiron@jmoiron.net"},
+			Created:     now,
+		},
+		&Item{
+			Title:       "Logic-less Template Redux",
+			Link:        &Link{Href: "http://jmoiron.net/blog/logicless-template-redux/"},
+			Description: "More thoughts on logicless templates",
+			Created:     now,
+		},
+		&Item{
+			Title:       "Idiomatic Code Reuse in Go",
+			Link:        &Link{Href: "http://jmoiron.net/blog/idiomatic-code-reuse-in-go/"},
+			Description: "How to use interfaces <em>effectively</em>",
+			Created:     now,
+		},
+	}
+
+From here, you can output Atom, RSS, or JSON Feed versions of this feed easily
+
+	atom, err := feed.ToAtom()
+	rss, err := feed.ToRss()
+	json, err := feed.ToJSON()
+
+You can also get access to the underlying objects that feeds uses to export its XML
+
+	atomFeed := (&Atom{Feed: feed}).AtomFeed()
+	rssFeed := (&Rss{Feed: feed}).RssFeed()
+	jsonFeed := (&JSON{Feed: feed}).JSONFeed()
+
+From here, you can modify or add each syndication's specific fields before outputting
+
+	atomFeed.Subtitle = "plays the blues"
+	atom, err := ToXML(atomFeed)
+	rssFeed.Generator = "gorilla/feeds v1.0 (github.com/gorilla/feeds)"
+	rss, err := ToXML(rssFeed)
+	jsonFeed.NextUrl = "https://www.example.com/feed.json?page=2"
+	json, err := jsonFeed.ToJSON()
+*/
+package feeds