Improve "Zoom to Selected" bounding box

[trvrb]

Context
The implementation of a "Zoom to Selected" button on the tree has been really useful. I'll often filter to say clade 21A and then be able to just click to zoom in properly.

However, there are a number of edge cases where "Zoom to Selected" isn't very helpful. For a simple example, if we filter to early samples (https://nextstrain.org/ncov/global?dmax=2020-02-01) there's no way to zoom into these (the button is deactivated). Or if we filter to clade 19A viruses (https://nextstrain.org/ncov/global?f_clade_membership=19A) there's also no way to zoom in.

Description
We could at some point contemplate a fancier "accordion" view which would help to emphasize situations where multiple blocks of tips are selected, but before this we should get basic bounding box working (the accordion view would require updating node y-values and will necessarily be more involved).

Instead, I'd recommend when clicking "Zoom to Selected" to calculate the maximum x and y-values and the minimum x and y-value out of all visible nodes. Use these values to define the bounding box to zoom to.

cc @joverlee521 who implemented the initial version of zoom to selected in #1257. This PR contains discussion of the implementation challenge of this bounding box design.

This implementation would also resolve #1360.