Have an App-like interface

[shish]

After getting an API which covers all of the features (#788), it would be nice to have an interface which is less "web-page-y" and more "app-y" (eg a JS single page app which only talks to the server via the API, maybe leading to native apps later?)

(This is in contrast to, though could be done in parallel with, #793)

[shish]

I'm pretty out of touch with JS app frameworks

• I've used HyperApp and loved it (eg https://scuba.shish.io/), being very simple and straightforward and free from overhead and typical JS bullshit
• I've also tried React and spent several hours trying (and failing) to make the "hello world" example compile, so my first impressions are poor... though it does look like it might be more appropriate for Shimmie's architecture (eg React's Components could be vaguely analagous to Shimmie's Blocks, where different pages have different components depending on which extensions are active)

(Though if the API and the frontend are sufficiently decoupled, there's no need to mandate a specific framework - we could have eg /themes/hyperapp/ and /themes/react/ side-by-side with no problems, the only limit being manpower to write different interfaces)