Manually extract brand_redirect from 1b569bbc99

This commit manually extracts the brand_redirect function from the
commit mentioned.

However, the redirect to the  `.../about` endpoint is removed due to the
fact that it doesn't exist yet.

This commit is also mainly just a bridge for the next few cherry picks from
\#2215

src/invidious.cr

@@ -315,6 +315,10 @@ Invidious::Routing.get "/channel/:ucid/playlists", Invidious::Routes::Channels,
 Invidious::Routing.get "/channel/:ucid/community", Invidious::Routes::Channels, :community
 Invidious::Routing.get "/channel/:ucid/about", Invidious::Routes::Channels, :about
 
+["", "/videos", "/playlists", "/community", "/about"].each do |path|
+  Invidious::Routing.get "/c/:user#{path}", Invidious::Routes::Channels, :brand_redirect
+end
+
 Invidious::Routing.get "/watch", Invidious::Routes::Watch, :handle
 Invidious::Routing.get "/watch/:id", Invidious::Routes::Watch, :redirect
 Invidious::Routing.get "/shorts/:id", Invidious::Routes::Watch, :redirect

src/invidious/routes/channels.cr

@@ -101,6 +101,34 @@ class Invidious::Routes::Channels < Invidious::Routes::BaseRoute
     env.redirect "/channel/#{ucid}"
   end
 
+  def brand_redirect(env)
+    locale = LOCALES[env.get("preferences").as(Preferences).locale]?
+
+    user = env.params.url["user"]
+
+    response = YT_POOL.client &.get("/c/#{user}")
+    html = XML.parse_html(response.body)
+
+    ucid = html.xpath_node(%q(//link[@rel="canonical"])).try &.["href"].split("/")[-1]
+    if !ucid
+      env.response.status_code = 404
+      return
+    end
+
+    url = "/channel/#{ucid}"
+
+    location = env.request.path.lchop?("/c/#{user}/")
+    if location
+      url += "/#{location}"
+    end
+
+    if env.params.query.size > 0
+      url += "?#{env.params.query}"
+    end
+
+    env.redirect url
+  end
+
   private def fetch_basic_information(env)
     locale = LOCALES[env.get("preferences").as(Preferences).locale]?