This also moves the CLJS output out of `frontend/src/cljs{,_release}` to
`target/cljs_{dev,release}`. If that causes problems for FE tooling I
don't know about, it could be moved back.

Hot reloading:
CLJS changes are hot-reloaded into the running app by Shadow CLJS - no
action is needed. Note that the UI will not refresh; you usually have to
click around so the updated MLv2 code gets executed.

Usage of the CLJS REPL:
- Open the browser (you should see `shadow-cljs: #N ready!` in console)
- Connect to nrepl on 50655
- "Select" the `:app` build;
    - https://shadow-cljs.github.io/docs/UsersGuide.html#_editor_integration
    - Conjure: `:ConjureShadowSelect app`
    - Plain REPL: Eval in CLJ mode `(shadow/repl :app)`
- Consider running `yarn build-hot:cljs` and `yarn build-hot:js` in
  separate terminals; then when `yarn build-hot:js` crashes you don't
  have to reconnect your REPL.
- The REPL works beautifully, even load...