Feature: template and theme override

[maartenbreddels]

Implements #635 Replaces #629, #201 and #258

This allow overriding the template and theme, e.g.:

http://localhost:8866/voila/render/basics.ipynb?voila-template=material&voila-theme=dark

Or via notebook metadata:

....
 "metadata": {
  "voila": {
    "template": "material",
    "theme": "dark"
  },
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
....

Configuration to disable this:

$ voila notebooks/ --VoilaConfiguration.allow_template_override=NO  # never allow this
$ voila notebooks/ --VoilaConfiguration.allow_template_override=NOTEBOOK  # only via notebook metadata
$ voila notebooks/ --VoilaConfiguration.allow_theme_override=NOTEBOOK  # similar for theme

[maartenbreddels]

Note that this PR also changes the static URL to include a hash, like `voila.js?v=ae356' such that we don't get bitten by aggressive caching. This is a feature of tornado, that also the notebook uses, and we simply didn't make use of.

[jtpio]

Wondering whether something like ['url', 'notebook', 'all'] would sound more explicit?

[maartenbreddels]

My reasoning was that you probably don't want the combination of 'permit from URL', but not from notebook metadata. Or can you come up with te use case?

[jtpio]

It was mostly for users who want to set the option without reading the docs or the help string, to know what 'YES' and 'NO' mean in this case.

[jtpio]

Nice, thanks @maartenbreddels 👍

We will probably want to add a quick word about this feature in the docs too.

[maartenbreddels]

Closes #105