Linting and Code formatting

[LlGC-jop]

[demiankatz]

Note that the project currently uses tslint and has tslint.json configuration. For minimum disruption, it would probably be good to see if there's a way to translate this into ESLint settings -- I imagine there probably is.

We should also keep in mind that some of the projects in IIIF Commons also use tslint; if we do the work to translate and modernize here, we should apply it to those other projects as well for consistency (and to make use of our knowledge before we forget it).

Regarding the HTML docs, those are primarily tied to configuration, so maybe their future should be considered as part of #1377.

[demiankatz]

Nearly done -- closing because outstanding work is tracked by #1485, #1486 and #1487 and this will make life less confusing.