Integrate with Ole's (external) Reveal nlesc template

[raar1]

To prevent divergence of the nlesc slide styles in use with NEBULA, and also to make the style loading modular (i.e. other users of NEBULA will be able to load their own styles more cleanly/easily) we are in the process of integrating with https://github.com/OleMussmann/reveal_template

We have a working attempt to do so by putting Ole's js into a reveal.js plugin. The WIP branch can be found here: https://github.com/OleMussmann/reveal_template/tree/convert_to_reveal_plugin

Once refactored it could probably best be used in NEBULA by importing it as a node package.

[raar1]

The test branch for this on NEBULA is here: https://github.com/esciencecenter-digital-skills/NEBULA/tree/reveal-escience-theme (but don't merge that, it's only for testing out if the plugin idea would work)