Workspace: New styling

This minor rework has two goals:

More obviously differentiate the workspace pages from the project pages (so it’s always clear where you are)
Pave the way for the upcoming workspace Focus feature

Steps

~~Workspace header pattern~~
~~Update workspace layout (including header and nav)~~
~~Page: Projects~~
~~Page: Updates~~
~~Page: Members~~
~~Page: Settings~~
~~Page: Confirm delete~~
~~Remove workspace summary/description (not essential)~~