This commit fixes the following warnings for the web build:
```
Warning: In index.html:38: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead.
Warning: In index.html:60: "FlutterLoader.loadEntrypoint" is deprecated. Use "FlutterLoader.load" instead.
```
To re-add the slash screen we alsow run the
`flutter pub run flutter_native_splash:create` command again, which also
updated the splash screens for iOS and Android.
Last but not least this commit also fixes the following warnings for the
Windows build:
```
windows/runner/flutter_window.cpp does not ensure the show window callback is called, updating.
windows/flutter/CMakeLists.txt does not use FLUTTER_TARGET_PLATFORM, updating.
```
- Adjust "Release" section in contributing guide.
- Disable "X" datasource, since it is not working after the latest API
adjustments.
- Update "version" key and "msix_config.msix_version" key in
"pubspec.yaml" file.
- Change user in "AddSourceReddit" widget help text.
- Enable macOS App Sandbox.
- Adjust logo for Windows version.
