added a new track for balancers

* as described in #1717
* this feature won't be available for testing until WS245 has been
  staged and the GBrowse configuration files built.
* See also related commits to the staging code in the admin repo
* Probably also requires changes to the Rearrangment summary, not yet specified.
Merge branch 'ws245_rearrangements' of github.com:WormBase/website into ws245_rearrangements