This change ports our two remaining legacy widgets (server cache and

activity graph) to inherit from BaseAdminWidget instead of the old

legacy Widget class, and then removes support for the legacy mode.
The server activity graph has additionally been ported to use the new JS

model/view architecture. As part of this, I've modernized the code a bit

but haven't spent any time redesigning the functionality or look.

Loaded the admin site and interacted with the widgets.