Unite PushNotificationSettingsScreen and NotificationSettingsUi

[FelberMartin]

Problem

There are currently two very similar composable functions PushNotificationSettingsScreen and NotificationSettingsUi, both for displaying the PushNotificationSettingsUi with a FAB button and triggering a save when the button is clicked. The first one is used for the settings screen (accessible by clicking on the settings icon on the top right), the latter appears when logging in the first time on a new server.

Suggested next steps

These two files can be merged and common logic should be extracted.

[julian-wls]

I think we can combine this issue and the missing save-dialog mentioned in #183 in one PR.