[#1163] Add optional banner to the plan list view

[vaszig]

No description provided.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (develop@0bd3c56). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop     #507   +/-   ##
==========================================
  Coverage           ?   96.53%           
==========================================
  Files              ?      521           
  Lines              ?    18781           
  Branches           ?        0           
==========================================
  Hits               ?    18131           
  Misses             ?      650           
  Partials           ?        0           

Impacted Files	Coverage Δ
src/open_inwoner/configurations/admin.py	100.00% <ø> (ø)
.../migrations/0034_siteconfiguration_plans_banner.py	100.00% <100.00%> (ø)
src/open_inwoner/configurations/models.py	98.47% <100.00%> (ø)
src/open_inwoner/plans/views.py	97.87% <100.00%> (ø)

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