Make transition from node selection to pinging smoother

[apike]

In #463 we have some visual mocks for a ping function. The PR #602 adds this functionality, but we should do a UI polish pass. Some priorities:

1. The average ping time should be shown in the prominent "slots" rather than the number of pings
2. The list of hosts you can ping should have lighter dividers, and be scrollable for if/when we add more hosts than can show on an iPhone SE
3. Ideally when you select a destination to ping, the popup for the node wouldn't appear with the traceroute button, disappear, then appear with the ping configuration.

[robmaceachern]

1 and 2 are being addressed in #604

I've spent some time trying to figure out 3 but it's still not clear exactly how to do this without having to reimplement a lot of the node selection/popover flow. The current implementation follows the same flow as traceroute (select the node and zoom in, show popover with node details, then start the action: traceroute/ping, zoom out, update the popover info). Reworking this flow is going to require substantial effort.

[apike]

Okay cool, I've renamed this issue to focus on the remaining thing and pushed it down in priority.