feature: initial syndication feeds support

Provides rss 2.0, atom 1.0 and json 1.0 feeds for videos (instance and account-wide) on listings and video-watch views.

* still lacks redis caching
* still lacks lastBuildDate support
* still lacks channel-wide support
* still lacks semantic annotation (for licenses, NSFW warnings, etc.)
* still lacks love ( ˘ ³˘)

* RSS: has MRSS support for torrent lists!
* RSS: includes the first torrent in an enclosure
* JSON: lists all torrents in the 'attachments' object
* ATOM: lacking torrent listing support

Advances #23
Partial implementation for the accountId generation in the client, which will need a hotfix to add a way to get the proper account id.

server/helpers/custom-validators/feeds.ts

@@ -0,0 +1,23 @@
+import { exists } from './misc'
+
+function isValidRSSFeed (value: string) {
+  if (!exists(value)) return false
+
+  const feedExtensions = [
+    'xml',
+    'json',
+    'json1',
+    'rss',
+    'rss2',
+    'atom',
+    'atom1'
+  ]
+
+  return feedExtensions.indexOf(value) !== -1
+}
+
+// ---------------------------------------------------------------------------
+
+export {
+  isValidRSSFeed
+}