Fix hard-coded LD zoom of 20kb

The LD code was hard-coded to export only start+20kb. This has been removed,
with the correct limits now coming from the Location object's coords. Links
in from a variation page already default to a 20kb window around the variant
so no need for the default limit to exist.
Merge pull request #1 from willmclaren/master

Fix hard-coded LD zoom of 20kb