add clean command in npm scripts

[hsiaoyi0504]

The idea of this is to have a npm script that can clean up files copied from node_modules folder to the place we want by webpack. This enhancement have at least following two benefits:

1. Reduce the complexity of developing front-end javascript/css based on new npm packages
2. boost the robustness of the deployment through deploy.sh

That is because currently we need to manually clean up files collected by webpack.

High level usage

Run this command: npm run clean.

When we want update (deploy) new genomics-workspace from one version to another version, the procedure will be following:

1. npm run clean
2. use git switch to newer version
3. npm run build
4. deploy.sh

Low level details

The current implementation in my mind is to have a script would remove files listed in .gitignore, but would not remove media folder.