Optimize admin queries

[jedie]

Contributed by https://github.com/henribru in #110

[codecov-commenter]

Codecov Report

Merging #114 (ed33679) into master (d077845) will increase coverage by 0.18%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   83.94%   84.13%   +0.18%     
==========================================
  Files          40       41       +1     
  Lines         841      851      +10     
==========================================
+ Hits          706      716      +10     
  Misses        135      135              

Impacted Files	Coverage Δ
huey_monitor/models.py	93.85% <ø> (ø)
huey_monitor/admin.py	86.04% <85.71%> (+0.86%)	⬆️
...tor/migrations/0010_alter_taskmodel_parent_task.py	100.00% <100.00%> (ø)

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