Fix update worker issues due to lack of old path redirect

Old UIs were still querying the old path (/update-worker.js) to refresh
their service worker, which no longer exists. The new path works just
fine, and is referenced correctly from the HTML, but wasn't accessed
until _after_ the UI had updated, leaving us in a bit of a bind.

This unties that knot with a proper redirect. We should probably leave
this in place for quite a while, since it's required for updates until
all UIs have reached this commit or later.

Caddyfile

@@ -3,6 +3,8 @@
 
     header *.wasm Content-Type "application/wasm"
 
+    rewrite /update-worker.js /ui-update-worker.js
+
     root * /site
     try_files {path} /index.html
     file_server

src/services/update-management.ts

@@ -1,5 +1,3 @@
-import dedent from 'dedent';
-
 import registerUpdateWorker, {
     ServiceWorkerNoSupportError
 } from 'service-worker-loader!./ui-update-worker';