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 * Remove outdated olm import code.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/indexeddb-worker.js

@@ -18,4 +18,4 @@ import {IndexedDBStoreWorker} from 'matrix-js-sdk/lib/indexeddb-worker.js';
 
 const remoteWorker = new IndexedDBStoreWorker(postMessage);
 
-export const onmessage = remoteWorker.onMessage;
+global.onmessage = remoteWorker.onMessage;