Put binder config in a different branch

[salvis2]

According to conversation here: NCAR/cesm-lens-aws#51 (comment)

the binder config should be on a different branch than the notebooks for easier CI work.

There is a problem I've seen with this, which is that on pangeo-gallery, the binder links on the left of the page when looking at individual notebooks have links that seem to default to the master branch. These should probably be removed so that the only way to launch the binder on the gallery site is from the base link at the specific gallery's root.