admin: use decorator syntax

[nijel]

Proposed changes

This makes the code easier to read.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• Other (please describe):

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (93f5127) 94.12% compared to head (17e54b6) 94.12%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #519   +/-   ##
=======================================
  Coverage   94.12%   94.12%           
=======================================
  Files          39       39           
  Lines        1140     1140           
  Branches      133      136    +3     
=======================================
  Hits         1073     1073           
  Misses         43       43           
  Partials       24       24           

Flag	Coverage Δ
unittests	94.12% <100.00%> (ø)

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Files	Coverage Δ
social_django/admin.py	97.05% <100.00%> (ø)

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