Back in Review Board 2.5, we began working toward a better standard for

how JavaScript-backed pages are defined, by way of RB.PageView,

RB.Page, and RB.PageManager objects. PageManager was responsible

for managing pages, but didn't really help with their construction (and

wasn't always used). RB.Page and RB.PageView were empty by default,

built for future expansion, but not very helpful up until now.
Over time, we started introducing concepts like full-page content mode

(used in places like the dashboard, where we want the main page content

to always be on-screen) and an action panel that overlays the sidebar.

These were implemented in the subclasses that needed them, making it

harder for new pages to leverage the capabilities.
This change fleshes out the base classes, giving them more consistent

responsibility over the page. RB.PageView now sets up the

RB.HeaderView (instead of leaving that to the template) and handles

the full-page content mode sizing and layout logic. It also takes care

of hiding and later showing the sidebar and content areas once initial

layout is complete, helping provide a smooth initial load of the page.
On a template level, we're now standardizing how page classes are set

up. Templates can provide the following blocks:

js-page-view-type: The RB.PageView class or subclass for the page.
js-page-view-options: A dictionary of options to pass to the view.
js-page-model-type: The RB.Page class or subclass for the page.
js-page-model-attrs: A dictionary of attributes for the model.
js-page-model-options: A dictionary of options for the model (useful

  for passing {parse: true}.

These get passed into a new RB.PageManager.setupPage() function, which

takes care of instantiating and setting the page in RB.PageManager.

As more pages make use of this, we'll be able to provide smarter desktop

and mobile page management.
For now, there will be conflicts with some pages. For instance, the

datagrid pages manually instantiate a subclass of RB.PageView, meaning

we'll end up with two of these. The code tries to work around this, and

warns to the console with advice. Upcoming changes will begin moving

pages over to all the new page and sidebar capabilities.

Tested all the current pages that provide a PageView subclass in both

desktop and mobile mode, making sure they work correctly and that there

aren't any JavaScript errors.
Updated some pages (which will be posted in an upcoming change) and tested

their behavior as well.