Labelset and label views: Javascript tests + modernizing

[StephenChan]

Tests should mostly be QUnit, with potentially a few LiveServerTestCases. Modernizing means:

• Using ES6 classes and syntax
• Not relying on jQuery too much, including using fetch instead of jQuery.ajax

4 files that I'd put under this issue:

• [ ] LabelList.js
• [ ] LabelMain.js
• [ ] LabelNew.js
• [ ] LabelsetAdd.js (the most complex one)

I'll group LabelsetImport.js along with upload (issue #436).