Admin UI, simplified and more error proof

The new edit UI gives a lot more feedback about problems, and makes it much more obvious what information is needed.

See commit comment for details.

This is still not enough to call #479 complete, since you still cannot run a query.
That's handled in `Confing.toFile`, when the file is written: https://github.com/slamdata/slamengine/commit/bccd718a5055ffd767c0974866b58887f339fa76#diff-5fcf5ece0aee1d4f824123689a3d11ceR77

I tested as many edge cases as I could think of, but this could certainly use some manual testing on all the platforms.