Add license processing

[christianliebel]

This PR introduces the npm run license-checker command, which would have to be run after new production packages are added (ideally, it’s run before a new deployment—see #144). The command creates a 3rdpartylicense.txt file with all npm production packages along with their licenses.

The licenses are shown in the about dialog.

NOTE: If https://github.com/1j01/jspaint/pull/144 should be merged, 3rdpartylicense.txt must be added to the workbox manifest.

Closes #63

[1j01]

• This includes the entire jspaint README, which I'm guessing would change to its LICENSE if it had one, but it's weird as it is.
• This doesn't include jQuery, or other libraries not versioned with npm
• This includes e.g. electron-is-dev but not electron, simply because it's in devDependencies (the electron stuff isn't really important, since the electron app isn't released, but this is an example of a discrepancy)
• I don't want a bunch of legalese in a box so that it's technically there but that you wouldn't actually want to look at.

I want to keep the about window clean, so this would probably be something in a separate window accessed with a button, and with expandos for each project so you can view it in a list, but optionally see the license texts by clicking on a project name.

Mainly tho, I want to personally understand the legal status of the project (is it using restrictive licenses?) so I know how I can (technically, legally) license it (even if that stuff doesn't practically matter).