Bump autoprefixer from 9.6.1 to 9.6.4 in /app (#228)

Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 9.6.1 to 9.6.4.
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/master/CHANGELOG.md)
- [Commits](postcss/autoprefixer@9.6.1...9.6.4)

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

Author: dependabot-preview[bot]

app/package-lock.json

@@ -14,7 +14,7 @@
     "@babel/plugin-proposal-class-properties": "^7.5.5",
     "@babel/preset-env": "^7.6.2",
     "@fullhuman/postcss-purgecss": "^1.3.0",
-    "autoprefixer": "^9.6.1",
+    "autoprefixer": "^9.6.4",
     "babel-loader": "^8.0.6",
     "css-loader": "^3.2.0",
     "file-loader": "^4.2.0",