Update mini-css-extract-plugin requirement in /{{cookiecutter.app_name}} (#522)

Updates the requirements on [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) to permit the latest version.
- [Release notes](https://github.com/webpack-contrib/mini-css-extract-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/mini-css-extract-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/mini-css-extract-plugin/compare/v0.7.0...v0.8.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

{{cookiecutter.app_name}}/package.json

@@ -43,7 +43,7 @@
     "less-loader": "^5.0.0",
     "manifest-revision-webpack-plugin": "^0.4.0",
     "less": "^3.9.0",
-    "mini-css-extract-plugin": "^0.7.0",
+    "mini-css-extract-plugin": "^0.8.0",
     "raw-loader": "^3.0.0",
     "url-loader": "^2.0.0",
     "webpack-cli": "^3.3.2",