Draft: Click for neuroglancer

Fixes #39, but a rough draft.

This removes the neuroglancer button in the top left, and puts a

on top of the neuroglancer iframe, which when clicked hides the div and shows neuroglancer. Someone else definitely needs to test this, but I think this delays loading neuroglancer until the

is clicked.

Styling suggestions very welcome too. Maybe bigger text? Maybe we should move the box above the first dataset table, so all the dataset tables are together at the bottom, and the visualisation is at the top?