The user infobox includes a nifty on-hover panel for the timestamp,

showing the last logged in time and joined date. The implementation was

a bit specific to the user infobox (partly due to the way it was

positioned and the hover triggered), but this change makes it more

generic and usable for other infoboxes, allowing it to be placed

anywhere in the infobox.
The new implementation provides standard CSS rules and a structure for

the hover panel. There's now a containing element

(.infobox-hover-item) with an anchor that triggers the display

(.infobox-hover-item-anchor) and the panel contents that will be shown

(.infobox-hover-item-contents). Unlike the previous implementation,

this one doesn't absolutely position the main container, meaning that it

can sit directly above and below other content without affecting the

flow of those elements.
There's also a .infobox-item class for items that don't have a hover

rule. This enables us to have consistent spacing for all detail items.
With the new implementation, we can no longer depend on :hover (since

the element used for it is now positioned relatively alongside the panel

instead of absolutely inside of it, and the panel is shown/hidden based

on the hover instead of its child elements). Instead, the hover

functionality now lives in JavaScript, and is common to all infoboxes.

In theory, infoboxes could have multiple of these hover panels and

they'd all work.

Tested that the user infobox's hover panel worked, and continued to work

as I switched between users and re-rendered the infobox.
Tested these CSS rules with new infobox content for review requests

(coming in another change).