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* Remove outdated olm import code.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.
4274c7287a