A number of improvements to notifications

[micahmo]

Pull Request Description

This PR adds a number of improvements to the notifications workflow.

• There is now a second line to the notifications setting subtitle that can help us to identify the current state of things.
  • When using local notifications, it says if we don't have OS permissions. (You could do something similar for APN/iOS.)
  • When using UnifiedPush, we will display whether we were able to find a compatible app or not with the following logic.
    • If no apps are found, a message to that effect is displayed.
    • If we find 1 app, but aren't connected yet, tell the user to restart.
    • If we found more than 1 app, tell the user we don't support that (we would have to present a selector dialog to them, which we can add later).
    • Otherwise, we tell the user what app we are connected to.
• When the user denies OS notifications, the snackbar now presents an option to navigate to system settings for them to enable it directly (in case they made a mistake).
• As part of the subtitle when selecting UnifiedPush, suggest and link to ntfy in F-Droid store.
• Updated the error message for disabling notifications to indicate it's really a problem with the server.
• I removed the instance name from the UnifiedPush registration. For some reason, with an instance name, I never got any callbacks (onNewEndpoint, onMessage) and getDistributor() always returned null. The registration is still unique to our app, so I hope we don't need the instance feature.

Everything here should be covered in the demos below.

Screenshots / Recordings

OS Permissions Denied

notif-not-allowed.mp4

OS Permissions Redirect

system-settings-redirect.mp4

UnifiedPush Flow

unified-push-flow.mp4

[hjiangsu]

I only briefly took a look at this. I'm fast-tracking this PR (and other notification PRs) so hopefully fix the issues that some have seen with notifications not working!