Build admin dashboard page

The plan for the dashboard page is to have it as an open space for widgets that can be added by extensions and customised by the user:


What needs to be done:
- Come up with a base widget component or interface that extensions can extend to define their own widgets.
- Allow extensions to register their widgets on `app.widgets` or `app.widgetComponents` or something.
- Render widgets on the DashboardPage, based on the value of a config setting that stores the widget layout.
- Allow widgets to be added/removed/dragged around in the UI, and save the layout to that config setting.

Start by locating DashboardPage and the extension registration/configuration points represented by app.widgets, app.widgetComponents, and the widget layout setting. Review how extensions and dashboard state are currently handled before defining the widget interface and UI behavior; done means extensions can register widgets, users can add, remove, rearrange, and customise them, and the layout persists.

Written by the indexing model from the issue text.