Store selection state in URL hash

[andy-sweet]

Closes #49
Closes #62 (by making it no longer relevant)

This stores selection state in the URL hash. In order to make this work, we need some state that reflects the selected points/cells/tracks independent of PointCanvas.curTime because the ordering of the React effects when re-hydrating from the entire state can be different compared to when the user is interacting with the application.

Previously, the canvas stored the last selected point indices, which is transient state that depends on PointCanvas.curTime. Here we stored the all the point IDs that were selected by the user (inspired from #93). Alternatively, we considered storing the track IDs that were selected in #91 , but that doesn't work as well with the rest of the application logic.

This PR also updates the URL hash to store some simpler state that not been added like point brightness. It also refactors that code a little to make it easier to keep new state in sync. Though there is still a lot of duplication that could be improved.

It does not fix #30 , which is related, but can be solved independently.

[aganders3]

This looks good and works in my manual testing, with no blocking bugs. I think we're on the same page about refactoring, etc. so I am happy to see this merged to move us toward feature-complete.