Initialize Webpack Plugins only once

When a plugin is defined in custom Webpack Config that is already defined in common.webpack.js it is treated as two seperate plugins that are both being initialized.
With this change this behaviour changes the following way:
When a plugin is defined in custom webpack config that is already defined in common.webpack.js it is treated as one plugin that takes the default config and merges config of custom Webpack-Plugin onto it.

This makes it possible to overwrite certain behaviour of frontend-scripts.