Fix missing .json file extension when exporting messages

Very similar to FossifyOrg/Notes#14 (which has a more detailed commit
message). Basically, some file pickers don't automatically append the
extension, so we should do it ourselves. It doesn't cause problems.

Reported in #88.

app/src/main/kotlin/org/fossify/messages/activities/SettingsActivity.kt

@@ -106,7 +106,7 @@ class SettingsActivity : SimpleActivity() {
     private fun setupMessagesExport() {
         binding.settingsExportMessagesHolder.setOnClickListener {
             ExportMessagesDialog(this) { fileName ->
-                saveDocument.launch(fileName)
+                saveDocument.launch("$fileName.json")
             }
         }
     }