Add hooks to load scripts

[juliusknorr]

As discussed with @rullzer we should add some hooks to load scripts depending on if the user is on a public page or a logged in one.

E.g. the firstrunwizard doesn't need to load any scripts on the public pages.

[rullzer]

so the events. I think it makes most sense to bind them to OCP\AppFramework\Http\TemplateResponse

Then we'd have

OCP\AppFramework\Http\TemplateResponse::loadAdditionalScripts - generic always
OCP\AppFramework\Http\TemplateResponse::loadAdditionalScriptsLoggedIn - only called when logged in

I think those two should be enough to start with right?