Move all database queries off the ui thread & add a ViewModel for MainActivity (#4786)

- Move all database queries off the ui thread - this is a massive
performance improvement
- ViewModel for MainActivity - this makes MainActivity smaller and
network requests won't be retried when rotating the screen
- removes the Push Notification Migration feature. We had it long
enough, all users who want push notifications should be migrated by now
- AccountEntity is now immutable
- converted BaseActivity to Kotlin
- The header image of Accounts is now cached as well

app/src/main/java/com/keylesspalace/tusky/receiver/NotificationBlockStateBroadcastReceiver.kt

@@ -20,7 +20,7 @@ import android.content.BroadcastReceiver
 import android.content.Context
 import android.content.Intent
 import android.os.Build
-import com.keylesspalace.tusky.components.systemnotifications.canEnablePushNotifications
+import com.keylesspalace.tusky.components.systemnotifications.isUnifiedPushAvailable
 import com.keylesspalace.tusky.components.systemnotifications.isUnifiedPushNotificationEnabledForAccount
 import com.keylesspalace.tusky.components.systemnotifications.updateUnifiedPushSubscription
 import com.keylesspalace.tusky.db.AccountManager
@@ -45,7 +45,7 @@ class NotificationBlockStateBroadcastReceiver : BroadcastReceiver() {
 
     override fun onReceive(context: Context, intent: Intent) {
         if (Build.VERSION.SDK_INT < 28) return
-        if (!canEnablePushNotifications(context, accountManager)) return
+        if (!isUnifiedPushAvailable(context)) return
 
         val nm = context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager