Use a dedicated endpoind for downloads

This allows us to not pass file name ("title") in the form
data and to enforce some sanity checks

src/invidious.cr

@@ -236,6 +236,7 @@ before_all do |env|
             "/api/manifest/",
             "/videoplayback",
             "/latest_version",
+            "/download",
           }.any? { |r| env.request.resource.starts_with? r }
 
   if env.request.cookies.has_key? "SID"
@@ -348,6 +349,8 @@ end
   Invidious::Routing.get "/e/:id", Invidious::Routes::Watch, :redirect
   Invidious::Routing.get "/redirect", Invidious::Routes::Misc, :cross_instance_redirect
 
+  Invidious::Routing.post "/download", Invidious::Routes::Watch, :download
+
   Invidious::Routing.get "/embed/", Invidious::Routes::Embed, :redirect
   Invidious::Routing.get "/embed/:id", Invidious::Routes::Embed, :show