Only show top-banner if no left-banner set

[knolleary]

Part of #8196

This updates the account create page to only show the top-banner if:

• in the popup window context
• otherwise, hide if left-banner has been configured

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.76%. Comparing base (43afc17) to head (3d3f023).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8360   +/-   ##
=======================================
  Coverage   76.76%   76.76%           
=======================================
  Files         459      459           
  Lines       24687    24687           
  Branches     6566     6566           
=======================================
  Hits        18951    18951           
  Misses       5736     5736           

Flag	Coverage Δ
backend	76.76% <ø> (ø)

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