Add a Zebrahub status indicator

[andy-sweet]

Occasionally, the Zebrahub data may not be available (e.g. due to maintenance). We should find a place to put an indicator to distinguish this type of problem from a client side visualization bug. This is mostly for early feedback and demos from/by non-developers.

[aganders3]

Right now the URL input is meant to turn red when it can't find an array. This is insufficient in practice because I think to "find" an array it just needs the .zarray file, which is often cached.

We might be able to combine this with a loading indicator, which I think we had at P0.