Make DAVDroid / DAVx5 integration more seamless

* clicking the "setup sync" button it should:
* automatically generate an app password and doesn't ask the user to type in the password in DAVDroid again
* automatically set the users email address instead of asking the user for it (even if prefilled)
* by default sync all calendars and address books and don't require the user to tick all those items manually

@tobiasKaminsky @AndyScherzinger Is this possible? This would improve the UX a lot 😃

cc @mario