Simplify and shrink the admin sidebar.
Review Request #10564 — Created May 24, 2019 and submitted
The sidebar in the admin site had grown pretty unweildy, including a lot
of information that really didn't need to be shown all of the time. This
change simplifies it a lot, eliminating the "System Information" box
completely and shrinking the width down to fit the content.
The one part of the System Information box which really still needed to
be linked was the security checklist, which has been added into the top
("System Settings") box and renamed as "Security Center".
Verified the appearance of the sidebar.