Use html-webpack-plugin for minifying HTML in production

[lslezak]

Problem

• The HTML file is not minified in production
• This is related to PR Load Agama translations #677 which added a quite big comment into the index.html file

Solution

• As the index.html is not that big it is not a major problem but IMHO it is a good idea to do it anyway
• Additionally it supports bundling the favicon so we can remove our hack, the favicon tag is injected by the plugin automatically

Testing

• Tested manually, in the production mode the comments and the extra spacing is removed, in development mode the code is unchanged
• In both cases it injects the favicon tag properly

[coveralls]

coverage: 72.051%. remained the same when pulling b00fb3c on minify_html into e8f9b93 on master.

[lslezak]

Note: Let's merge this one after releasing the Dolomite image.