Instance/Static runtime plug

This allows to set-up an arbitrary directory which overrides most of the
static files: index.html static/ emoji/ packs/ sounds/ images/ instance/
favicon.png.

If the files are not present in the directory, the bundled ones in
priv/static will be used.

config/config.exs

@@ -110,6 +110,7 @@ config :pleroma, :instance,
   public: true,
   quarantined_instances: [],
   managed_config: true,
+  static_dir: "instance/static/",
   allowed_post_formats: [
     "text/plain",
     "text/html",