`npm run build` removes needed scripts

[bdeshi]

javascript files for various widgets (like asciinema, mermaid etc) are only kept in static/assets/js.

But the default build command (NODE_ENV=production webpack) first deletes everything from that folder, so those widgets don't work anymore afterwards.

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these are deleted as of 97989c26525bcf3d08fdc8508903c67780f9145b:

deleted:    static/assets/js/Chart.js
deleted:    static/assets/js/WaveDrom.js
deleted:    static/assets/js/asciinema-player.js
deleted:    static/assets/js/flowchart-latest.js
deleted:    static/assets/js/mermaid.min.js
deleted:    static/assets/js/narrow.js
deleted:    static/assets/js/raphael.min.js
deleted:    static/assets/js/sequence-diagram-min.js
deleted:    static/assets/js/snap.svg-min.js
deleted:    static/assets/js/underscore-min.js
deleted:    static/assets/js/viz.js
deleted:    static/assets/js/viz.render.js
deleted:    static/assets/js/webfont.js