This is a large cleanup of our infobox code. Originally, we only had

infoboxes for users, and they didn't need to do anything other than

display some HTML. Over time, this became more complex. The user infobox

had more interaction requirements with its date/time display, and we

added new types of infoboxes, all sharing the same logic.
Infobox display management was left up to each infobox instance (of

which there was one per type of infobox). While you'd never see more

than one instance of a type of infobox on the screen at the same time,

it was possible to get two infoboxes of two different types to appear at

the same time.
This change separates out infobox display management/target registration

from the infoboxes themselves. There's now a RB.InfoboxManagerView

that manages each type of infobox on the page. It keeps track of the

active infobox, shows/hides based on the mouseenter and mouseleave

events (along with the smarts for allowing the user to move the mouse

from the target element to the infobox without causing anything to

hide), and handles creating/populating the instances of the infoboxes.
It also has a utility function for creating the $.fn.* methods we use

to register infoboxes on elements, like those with the .user class.

This allows us to maintain that compatibility and ease of use in a more

straight-forward way.
The infobox views themselves handle the actual rendering and interaction

for the infobox content. There's a subclass of RB.BaseInfoboxView for

each type of infobox we use, along with a jQuery $.fn.* registration

for each (which allows us to remove some more code from common.e6.js).
This is the first part of a larger cleanup that will lead to more

reusable infoboxes, and even allow for extensions to more easily provide

their own infoboxes.

Tested the dashboard, All Review Request page, Groups/Users pages, and

the review request page to check each type of infobox.
Tested moving between valid targets of the same type and of different

types to make sure that infoboxes displayed and transitioned corerctly.
Tested moving between the target and the infobox or vice-versa kept the

infobox on-screen without triggering a new transition.
Tested that previously-cached infobox contents were shown immediately on

hover.
Tested that the user infobox's date/time handling (both the "timesince"

filter and the hover display on the time) were working correctly.
New unit tests pass.