Draft: Yaffol/frontend#4 typescript

This is the conversion of the Auth.js authentication helper class, which is used to communicate in the authentication frontend on doi.crossref.org to communicate with the authentication API. It's quite rudimentary, and the scope has been deliberately limited to just the Auth.js helper where possible. Some changes were needed to the library's bootstrapping (essentially the configuration and 'what environment am I deployed in?' parsing).

There are other areas of the project (ie the whole project, with all the Vue.js components) that could do with being converted, and providing module definitions for the library would make this a better overall implementation.

The unit test for Auth.js has been converted as well, and the ts-jest package used to enable typescript support within the Jest test runner.

Some linting fixes have crept in to other files - sorry about that!