Extremely minor remote dataset UI improvement suggestion

[pgunn]

Neuroglancer URLs to datasets look like this:

precomputed://https://s3.us-east-2.amazonaws.com/SOMETHING

It'd be cool if, during remote dataset import, if there's a precomputed:// in front, it's chopped off before parsing the rest, just to remove a step (where people may make mistakes) from moving data between.