Fix the IndexedDB worker

Looks like this was broken in the webpack 4 upgrade due to the
worker script setter and the bundle being re-ordered in index.html.

 * Remove the loop: we only use two scripts now, so import them
   explicitly
 * Remove outdated olm import code.
 * Stop generating a script import for each theme: we were pulling
   in 3 js files that did absolutely nothing.
 * Fix worker 'onmessage' scope (set it as a global rather than
   trying to make it an ES6 module which it isn't).
 * Fail hard if the indexeddb worker script isn't set to avoid
   this happening again.

src/vector/index.js

@@ -207,6 +207,13 @@ function onTokenLoginCompleted() {
 }
 
 async function loadApp() {
+    if (window.vector_indexeddb_worker_script === undefined) {
+        // If this is missing, something has probably gone wrong with
+        // the bundling. The js-sdk will just fall back to accessing
+        // indexeddb directly with no worker script, but we want to
+        // make sure the indexeddb script is present, so fail hard.
+        throw new Error("Missing indexeddb worker script!");
+    }
     MatrixClientPeg.setIndexedDbWorkerScript(window.vector_indexeddb_worker_script);
     CallHandler.setConferenceHandler(VectorConferenceHandler);