This replaces the old JavaScript code for the admin dashboard with a

new, simpler, and cleaner implementation built on top of our

RB.Page/RB.PageView classes. The new implementation isn't limited to

two columns of Large/Small widget sizes, but rather allows utilitizes as

many columns as can fit on the page.
The new page makes use of a modern version of Masonry, which is what we

use for laying out widgets. This version is a bit smarter with how it

handles layout and gives us better control over its layout rules.
The new layout rules may lay out widgets in a way that seems unfamiliar

when used to the old dashboard design. There's no longer a dedicated

column of smaller widgets, though that column will seem to appear

depending on the width of the window. As widgets are rewritten, they may

be adjusted to even out the sizing of widgets to help keep things more

uniform.
The dashboard no longer allows widgets to be added, deleted, or moved.

We might bring this back someday, but it would be with an entirely

different implementation. It's unlikely anyone really made use of this

in the past, though, and it's probably better to bring this back to

basics for now.
Upcoming changes will redo the implementation of widgets and widget

registration to fix issues with responsive layout and in widgets and to

make it easier to perform some rewrites of these widgets.

Tested the layout at various window sizes, and in mobile mode.