Add app version to JS and CSS

This leads to the regeneration of the hash in case a single application is updated. - This does not touch the Assetic Pipeline because I don't know how it works.

Fixes https://github.com/owncloud/core/issues/11374

@karlitschek Shall we backport this?
Mhm, actually I don't know how we should get informed about new app versions then... - We therefore should better invest time to cache this action inside `OC_App`... - Let's make this in another PR.