Move update notification code into app

Moves the update notification code in a single app. This is required since we want to use SSO for the new updater and for this have some code running in ownCloud as well (and we don't want that in core neccessarily). This app can provide that in the future, right now it's only the update notification itself. Will continue working on the SSO right away but wanted to keep the PR small.

Furthermore also makes some more code unit-testable...