Generate CSS filenames with contenthash

Previously, .css files always had the same name, which can lead to caching problems.
In our case, the new CSS for the HedgeDoc logo was not loaded when Chrome had the 1.6.0 CSS in the cache, leading the HedgeDoc logo filling the whole screen.
This commit adds the contenthash to the .css files generated by webpack, which ensures that changed files are always loaded.

References:
https://github.com/webpack-contrib/mini-css-extract-plugin#filename
https://webpack.js.org/configuration/output/#outputfilename
Signed-off-by: David Mehren <git@herrmehren.de>

webpack.common.js

@@ -177,7 +177,10 @@ module.exports = {
         }
       ]
     }),
-    new MiniCssExtractPlugin()
+    new MiniCssExtractPlugin({
+      filename: '[name].[contenthash].css',
+      chunkFilename: '[id].[contenthash].css'
+    })
   ],
 
   entry: {