Assets minification and revision in build task

Now the build task minimizes and adds a revision hash to the js asset. If we'll have extra files for this, like images and such, we need to modify this an calculate the hashes for them as well.

To avoid the chicken and egg problem of the hash being dependant on the contents of the files and the files needing to set the script name in a script tag, I calculated a rev hash in advance and pass it to the webpack build using the `DefinePlugin` and as a global variable in the static generator script.