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

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

options/locale/locale_en-US.ini

@@ -228,6 +228,7 @@ view_home = View %s
 search_repos = Find a repository…
 filter = Other Filters
 filter_by_team_repositories = Filter by team repositories
+feed_of = Feed of "%s"
 
 show_archived = Archived
 show_both_archived_unarchived = Showing both archived and unarchived
@@ -2777,6 +2778,8 @@ publish_release  = `released <a href="%s/releases/tag/%s"> "%[4]s" </a> at <a hr
 review_dismissed = `dismissed review from <b>%[4]s</b> for <a href="%[1]s/pulls/%[2]s">%[3]s#%[2]s</a>`
 review_dismissed_reason = Reason:
 create_branch = created branch <a href="%[1]s/src/branch/%[2]s">%[3]s</a> in <a href="%[1]s">%[4]s</a>
+stared_repo = stared <a href="%[1]s">%[2]s</a>
+watched_repo = started watching <a href="%[1]s">%[2]s</a>
 
 [tool]
 ago = %s ago