Investigate errors

[evilkermit]

Three odd behaviors with the matplotlib plots during:

1. Data loading got stuck in a seemingly infinite loop in the UB tab.
2. The slice view in the volume slicer tab "broke" with weird axes and wouldn't render anything.
3. There was a weird whitespace below the detector view.

[evilkermit]

I am going to look at swapping WebAgg for CanvasAgg. The downside is that we have to manually pass the images from the server to the canvas in the front-end in the nova-trame library. The upside is that we don't need to run the separate websocket server, which seems to be the source of the unreliability for most of these issues.

I think 3 is still an issue after this change, but I believe this can be fixed with nova-model/nova-trame#166.