Multi-mine data federation

[heralden]

This would include performing a keyword search across multiple mines, and possibly queries. The scope, how to make this reliable and performant, as well as integration into the existing UI will require lots of thoughts and experimentation.

We should also assess what parts of the federation would be better placed in the Intermine backend (particularly to make the functionality available to other API consumers), or if we can determine a scope to limit this to the frontend.