The old design for widgets wasn't very generally useful. The server-side

design of Widget was really only geared toward the use cases we

initially came up with, but wasn't optimal or future-proof. The

client-side was worse -- there was none. Widgets just dumped HTML with

<script> tags into the page, and just tried hard-coding DOM element

IDs as needed, using them to try to look up widgets and populate them.
This change completely redoes the design, while striving to maintain

some backwards-compatibility (until Review Board 5.0).
Server-side, widget support is now provided through a BaseAdminWidget

class, which provides:

Basic class attributes for customizing the widget's ID, name, CSS

  classes, and a template name
An optional custom template, which would subclass a base

admin_widget.html (instead of being embedded within the base

  template) and provide content and header/footer actions
A method for subclasses to provide context data for the template
Methods for specifying attributes and options for the JavaScript model

  class, and options for the view class
A method for rendering the template to a string

It doesn't manage caches anymore, as that's not always necessary or even

worth doing. It also doesn't have a concept of primary/secondary

widgets or hard-coded list of action URLs. The legacy Widget class

still provides all this, but is deprecated and will be removed in 5.0.
Widgets are now managed by a registry, rather than being stored in

separate primary/secondary lists of classes.
Client-side, RB.Admin.WidgetView handles all widget rendering and

interactions, with state stored in RB.Admin.Widget. The default

implementations do effectively nothing, but can be subclassed for custom

widget behavior.
Legacy widgets still work, and will continue to in 4.0, but we emit

warnings in Widget.__init__(), AdminWidgetHook, and the legacy

register_admin_widget() and unregister_admin_widget() functions to

encourage anyone actually writing custom widgets to upgrade.
Upcoming changes will port all existing widgets to the new classes.

Tested with a mixture of legacy widgets and some partially-ported

widgets (server-side and client-side). Everything registered and

rendered correctly.
Unit tests pass.