Add special toggle to choose minerva/system instance as source of model data

[dustine32]

From Alliance Pathways 2021-11-02 call, suggested by @cmungall.

Add a little toggle to the pathway viewer widget so that devs and curators can look at models in either prod or dev minerva instances. The toggle would likely go here:

This toggle should only be accessible on the sandbox instance available at http://proto.geneontology.xyz/

Tagging @kltm @tmushayahama