Ideas for additional diagnostics

Related #829

Just some ideas, not sure if those are even possible/wanted.

## General

- [ ] Make diagnostics exportable
- [ ] Allow access to log files (show/export?)

## Notifications

- [x] Allow sending test-notifications
Needs https://github.com/nextcloud/notifications/issues/1206
Related https://github.com/nextcloud/talk-android/issues/2112
- [ ] Subscribe to push-proxy if account is not registered?
- [ ] Any additional troubleshooting information about push(-proxy)
- [x] Check if notification-app is installed on the server

## WebRTC
- [ ] Is STUN reachable
- [ ] Is STUN working?
- [ ] Is TURN reachable
- [ ] Is TURN working?
- [ ] Is HPB reachable
- [ ] Is HPB working?

## Talk

- [x] Show version of Talk app
Need: https://github.com/nextcloud/spreed/pull/7489

## Signaling

- [x] Show version of external signaling server (take from `hello`?)