Optimize DB access in AccessTokenAdmin

[knaperek]

Description of the Change

This is needed to avoid 2 extra DB queries per each line in the list view, so that is usually +200 unnecessary queries.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #988 (49951a0) into master (6dbde71) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #988   +/-   ##
=======================================
  Coverage   96.55%   96.56%           
=======================================
  Files          31       31           
  Lines        1715     1716    +1     
=======================================
+ Hits         1656     1657    +1     
  Misses         59       59           

Impacted Files	Coverage Δ
oauth2_provider/admin.py	100.00% <100.00%> (ø)

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[auvipy]

CHANGELOG.md updated (only for user relevant changes) is not updated

[knaperek]

CHANGELOG is not updated as this is not a user relevant change. I marked that tickbox as having that step completed, that is, updated CHANGELOG in case of a user relevant change; which it wasn't, so this is a no-op :-)

[auvipy]

ok