Structure code into SDK

[enemeth19]

No description provided.

[kaczmarj]

this is a great idea imho

[kaczmarj]

i've been looking into eslint to lint/format our code (see #332), and i get the feeling that restructuring this codebase as an SDK (presumably with modules) would clean up the organization and make it more explicit where variables / functions are defined and used.

[kaczmarj]

i started doing a little work on this using ecmascript 6 modules. my thoughts so far:

• it doesn't seem like this can be an incremental change. in other words, we can't sprinkle in export and import statements little by little. given this, i think we will need to plan a time to do this. some PRs should probably be merged before this, like the localstorage caching work.
• there are some global state variable that i think should all go in one module (state.js). for example, this includes allClinicalData and perhaps the data for the select boxes. those variables are defined in the html but also referenced throughout the javascript code. it feels cleaner and more organized to me to put those stateful variables in one place.