Refactor Search to separate component for searching runs and component for drilling into nested catalogs

[danielballan]

The current implementation of the model Search could be described as:

• Search IS a model for drilling down into nested catalogs.
• Search HAS a model for searching catalogs of runs (RunSearch).

Proposal:

• Make RunSearch a separately useful component. It's only part of the way there at present.
• Make a separate model for drilling down into nested catalogs as a stand-alone thing.
• Combine by composition in a small "drill down and then search" model.

[danielballan]

As part of this work, make it possible to pass custom search fields into Search and thread them down to SearchInput. Currently there is no way to do this.