Refreshed Sourcegraph administration dashboard

We're reworking the Sourcegraph administration pages to make Sourcegraph easier to set up and maintain.

The overview page has been reworked to provide administrators with a quick overview of the instance, whether there are any notifications that need attention, and some recommendations on what they might want to view or set up next.

The sidebar is also now searchable with collapsible headings, making it much easier to find the page you're looking for.

Speaking of notifications, a new notifications widget will alert administrators when something requires attention.

This is the first of a set of improvements planned to make setting up and managing Sourcegraph a much more enjoyable experience.