Implement endpoint to open files in the browser

[juliusknorr]

Implements #2418

Adds the following entry to the richdocuments capabilities:

<editonline_endpoint>http://nextcloud.dev.local/index.php/apps/richdocuments/editonline</editonline_endpoint>

A file can be opened through: http://nextcloud.dev.local/index.php/apps/richdocuments/editonline?path=/Test.odt&userId=admin

• If the user is not logged in they will be redirected to the login page
• If the user is logged in they will see an error
• If the passed user id is different from the currently logged in user, they will see a hint about that