add impersonation styling to django admin

[nora-shap]

Purpose/Motivation

If you use the django admin site while impersonating, it gets ugly (becomes very visually apparent that you are in an altered state)

Links to relevant tickets

codecov/engineering-team#3465

preview from my local (the banner got cut off, it says Impersonating (return to codecov.io and log out)):

[seer-by-sentry]

✅ Sentry found no issues in your recent changes ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.87%. Comparing base (b4563c1) to head (6b90630).
Report is 8 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1214   +/-   ##
=======================================
  Coverage   95.87%   95.87%           
=======================================
  Files         493      493           
  Lines       16877    16877           
=======================================
  Hits        16180    16180           
  Misses        697      697           

Flag	Coverage Δ
unit	95.76% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

📢 Thoughts on this report? Let us know!