Fix static subresource paths for server/viewer

[paulirish]

Fixes #875

Basically a continuation from last year's #753 and more specifically #786

The short version is... esbuild consuming HTML and css is rough. Parcel treated us well.

in 786 we fixed static serving (the viewer) but made sure the express-based serving (the server) still worked.
it did until it didnt. maybe some dependency bumps? shrug.

But...... this works.
I spent a bunch of time making a more generalized solution. I ended up for something sliiiiightly more specific, but I'm still happy with it.

oh.. and just like in #786 ...

some development notes

of course the root problem is plenty of because paths. and express (sometimes) but not all the time.

i used these to iterate.

# from viewer folder
find ../../scripts/build-app.js ./src/ui | entr bash -c "rm -rf ../viewer/dist/*; yarn build; cat dist/index.html"
# from server folder
find ../../scripts/build-app.js package.json src/| entr bash -c "rm -rf ../server/dist*; yarn build:esbuild; bat dist/index.html"