[Bugfix] Improve OPML route security (#535)

* WIP - moved plugs; set up a new token-protected route plug

* Added a route_token column to settings model

* Hooked up token_protected_route plug to database

* Hooked up new OPML route to UI; turned RSS and OPML feed buttons into links

* Docs, tests

* Added a note about the phoenix bug

priv/repo/migrations/20241230192618_add_route_token_to_settings.exs

@@ -0,0 +1,11 @@
+defmodule Pinchflat.Repo.Migrations.AddRouteTokenToSettings do
+  use Ecto.Migration
+
+  def change do
+    alter table(:settings) do
+      add :route_token, :string, null: false, default: "tmp-token"
+    end
+
+    execute "UPDATE settings SET route_token = gen_random_uuid();", "SELECT 1;"
+  end
+end