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.