A slew of new tracks in support of the excellent work distinguishing variations.

Ticket remains open as there are many things still to consider but core aspects of #993
have been addressed.
Added a file of shared configuration for variation tracks.
Merge branch 'staging' of github.com:WormBase/website into staging
added a citation for the natural variation track

* #663 is nearly complete and should go live with 236
* a few issues remain, such as linking to external sources