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/ViewMediaActivity.kt

@@ -43,6 +43,7 @@ import androidx.lifecycle.lifecycleScope
 import androidx.viewpager2.adapter.FragmentStateAdapter
 import androidx.viewpager2.widget.ViewPager2
 import com.bumptech.glide.Glide
+import com.google.android.material.snackbar.Snackbar
 import com.keylesspalace.tusky.BuildConfig.APPLICATION_ID
 import com.keylesspalace.tusky.components.viewthread.ViewThreadActivity
 import com.keylesspalace.tusky.databinding.ActivityViewMediaBinding
@@ -91,11 +92,9 @@ class ViewMediaActivity :
             if (isGranted) {
                 downloadMedia()
             } else {
-                showErrorDialog(
-                    binding.toolbar,
-                    R.string.error_media_download_permission,
-                    R.string.action_retry
-                ) { requestDownloadMedia() }
+                Snackbar.make(binding.toolbar, getString(R.string.error_media_download_permission), Snackbar.LENGTH_SHORT)
+                    .setAction(R.string.action_retry) { requestDownloadMedia() }
+                    .show()
             }
         }