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/uuid.go

@@ -0,0 +1,27 @@
+package feeds
+
+// relevant bits from https://github.com/abneptis/GoUUID/blob/master/uuid.go
+
+import (
+	"crypto/rand"
+	"fmt"
+)
+
+type UUID [16]byte
+
+// create a new uuid v4
+func NewUUID() *UUID {
+	u := &UUID{}
+	_, err := rand.Read(u[:16])
+	if err != nil {
+		panic(err)
+	}
+
+	u[8] = (u[8] | 0x80) & 0xBf
+	u[6] = (u[6] | 0x40) & 0x4f
+	return u
+}
+
+func (u *UUID) String() string {
+	return fmt.Sprintf("%x-%x-%x-%x-%x", u[:4], u[4:6], u[6:8], u[8:10], u[10:])
+}