Refactor count properties to custom managers

[krzys-h]

This reduces the SQL queries required to load the 'Workshops' view significantly, and helps prevent ORM N+1 problems in the future

Closes #622

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This change is 

[codecov[bot]]

Codecov Report

Merging #623 (93cb956) into master (16337e9) will increase coverage by 0.55%. The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
+ Coverage   84.37%   84.92%   +0.55%     
==========================================
  Files          15       15              
  Lines        2451     2461      +10     
==========================================
+ Hits         2068     2090      +22     
+ Misses        383      371      -12     

Impacted Files	Coverage Δ
wwwapp/forms.py	96.07% <ø> (ø)
wwwapp/views.py	80.76% <73.33%> (+0.06%)	:arrow_up:
wwwapp/models.py	85.25% <100.00%> (+3.01%)	:arrow_up:

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