Devices can now subscribe to specific topics.

- Device can now become (in)active
- Error alert makes sound
- Alerts now execute function on click

android/app/src/main/java/de/nicolasklier/livebeat/MainActivity.kt

@@ -58,7 +58,7 @@ class MainActivity : AppCompatActivity() {
             val response = client.newCall(req).execute()
 
             if (response.code != 200) {
-                Snackbar.make(findViewById<FloatingActionButton>(R.id.fab), "Device isn't registered yet.", Snackbar.LENGTH_SHORT)
+                Snackbar.make(findViewById<FloatingActionButton>(R.id.fab), "Device isn't registered yet. Registering ...", Snackbar.LENGTH_SHORT)
                         .setBackgroundTint(Color.YELLOW)
                         .setTextColor(Color.BLACK)
                         .show()