Emit to load additionalscripts

Fixes #13662

This will fire of an event after a Template Response has been returned.
There is an event for the generic loading and one when logged in. So
apps can chose to load only on loged in pages.

This is a more generic approach than the files app event. As some things
we might want to load on other pages as well besides the files app.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>