Clickable notification title when click.url is set

**Is your feature request related to a problem? Please describe.**
If I send a notification with this extra set:
```json
{
"extras": {
"client::notification": {
"click": { "url": "https://gotify.net" }
}
}
}
```
the `url` parameter is only available on the Android app (only when the notification is pressed, then its value is lost, I may implement this on the Android side, too)

**Describe the solution you'd like**
Make (at least) the notification title clickable with the extra parameter in the JSON payload

**Describe alternatives you've considered**
Leave as it is

**Additional context**
I can send a PR