Improve creation of custom JS views and models for config pages.

ConfigPagesView now allows for creating a custom models and views for
the configuration page, along with custom options/attributes. These will
be outputted in the template when setting up the page.

A new template tag, json_dumps_items, helps to output data without
messing around with more expensive and messy template-based loops.
Add support for spinners for list items in configforms.

This brings over RBCommons's list item spinners, but makes their
presence dynamic. A ListItemView subclass can now call showSpinner() to
cause a spinner element to fade in, and hideSpinner() to fade it out and
remove it.
Provide the ConfigPagesView in the ConfigPage constructor.

It's useful for ConfigPage subclasses or their forms to have access to
central state computed for a ConfigPagesView. Now, ConfigPages are
provided with the view when being constructed. ConfigPages already pass
themselves to ConfigPageForms.
Add a base class for a collection of ListItems.

This adds ListItemsCollection, which subclasses Backbone.Collection to
provide an event when fetching and to make constructor options available
as a variable on the collection.

Views can use the new "fetching" signal to show a loading indicator when
fetching a collection from the server.