Move all persistent state from AppState to Settings

This way all the persistant state of the app is managed from the same
place. It makes everything much easier. Also, it's required for when
GitJournal supports multiple repositories.

lib/screens/settings_git_remote.dart

@@ -132,7 +132,8 @@ class _GitRemoteSettingsScreenState extends State<GitRemoteSettingsScreen> {
     }
 
     var stateContainer = Provider.of<StateContainer>(context);
-    var gitDir = stateContainer.appState.gitBaseDirectory;
+    final settings = Provider.of<Settings>(context);
+    var gitDir = settings.gitBaseDirectory;
 
     // Figure out the next available folder
     String repoFolderName = "journal_";