Wagtail-references

Closes 15

List of references in the index no longer stacks them horizontally (this was confusing) so each now appears on a new line.

Features:
Improved rendering of the references where they appear in a list. Rendering is now:
- **more robust** since an entire single reference is rendered in one include block. If rendering a single one goes wrong, the rest don't fail.
- **prettier** ``citation.js`` is used in the template to convert the bibtex to apa-style citations in HTML. So you get to see how they'll actually be rendered on the front end.

Docs:
Updated the readme to include usage examples:
- How to use ``citation.js`` in template rendering
- A react component that converts and displays the citation
- Removed the roadmap lines for things I just did! Yay!

Updated docs that were outdated by theh prior version.

Moved the bibtex parsing code out of a signal, and onto the model. Added model methods to generate slugs automatically and robustly, as well as synchronizing the citation key used in bibtex with the slug.

This refactor allows (and in future will allow even better) exception handling on form validation, so we can make the add/edit pages much more robust and user friendly (e.g. with relevant error messages explaining parsing errors) over time.

The add and edit forms have been adjusted, so that adding an entry does not require a slug (it is autogenerated from the content initially), but so that the autogenerated slug can be manually edited later.

A number of tests have been added, dramatically improving coverage from an appalling (30ish%) to a poor (56%).

This allows reference snippets to be created on the fly within a page or streamfield creation process, which is an extremely common use case... Let's face it, if you're editing a story in your CMS you want to be able to add references as you go, instead of leaving the view, creating the reference, then returning to add the new reference into your story.

The 'Orderable' base class was also added to the snippet. So if you were to use a list of references in another model, you'd be able to re-order them as you please.