Use ES6 module style to enable frontend unit tests

[mtlynch]

Splitting off from #538 (comment)

It would be nice if we could write unit tests for TinyPilot's frontend, which currently has no automated tests. To do this, we need to use ES6 module syntax so that we can run unit tests under Node and import our web components.

[jotaen]

Fixed via https://github.com/tiny-pilot/tinypilot-pro/pull/231.

We now have everything in place so that we can start writing unit tests in the frontend. I think good candidates to start with would be prettifyByteSize or futureLocation. (We would need to extract these beforehand, of course.)

@jdeanwallace fyi, we are using ES6 modules now everywhere in the frontend. The differences in a nutshell: the <script> blocks are all type “module” now. That means, that all dependencies must be imported, like the controller functions or the dialog events. We don’t have the global scripts anymore, since these files have been migrated to be modules. There is also no js/util folder anymore, but only one folder with all our importable JavaScript modules.