[#65] Remove deprecated app registration functionality

[marius-wieschollek]

This PR aims to remove any deprecated functionality and replace it with an up-to-date solution.

• The app.php is no longer used
• Personal settings are registered in the info.xml
• Stylesheets are now registered within a template event
• CSP is now registered within a csp event

Signed-off-by: Marius David Wieschollek git.public@mdns.eu