Movement of refget from hts-specs to GA4GH refget

[andrewyatz]

As part of approval for refget sequence collections, we have created a new repository for refget under the GA4GH GitHub space. This PR denotes the current supported location is in the new location but retains the v2.0.0 spec here for reference.

[jkbonfield]

We should probably remove it from the README file (or point it elsewhere) and similarly the http://samtools.github.io/hts-specs/ site too.

[jmarshall]

If refget wishes to move, then it should do so properly IMHO and remove the soon-to-be-outdated text. I have activated jekyll-redirect-from in this repository, so (after rebasing on current master) you can replace all of refget.md with simply

---
redirect_to: https://ga4gh.github.io/refget/sequences/
---

You should also either remove the refget paragraph in README.md or (better) change the link and add text saying that it is now maintained elsewhere; remove the refget line in index.md; delete pub/refget-openapi.yaml and pub/ga4gh_and_TRUNC512_identifiers.*. You may also wish to remove the refget entry from MAINTAINERS.md.