Add support for notification channels

[cketti]

To be able to target API 26 we need to add support for notification channels.

For a minimal implementation we probably want to create a notification group for each account and then create two notification channels for each group. One for new message notifications and one for all other notifications (error notifications etc).
When using notification channels we can only provide the initial configuration. To change the ringtone, vibration, priority of an existing notification channel the user has to go to the Android settings app. So for API 26+ devices we probably want to hide those notification settings and display a link to Android's notification settings screen for K-9 Mail instead.