fix: count distinct registrants to avoid duplicates

[walidmujahid]

Description

HackathonRegistration instances are prone to have duplicates as described here #7070. This pull requests ensures that hacker_count counts only distinct registrants for a hackathon event. Tested locally.

Refers/Fixes

fixes #7051
related #7070

[codecov]

Codecov Report

Merging #7071 into master will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7071      +/-   ##
==========================================
+ Coverage   26.30%   26.32%   +0.02%     
==========================================
  Files         301      301              
  Lines       29241    29241              
  Branches     4295     4295              
==========================================
+ Hits         7691     7697       +6     
+ Misses      21279    21273       -6     
  Partials      271      271              

Impacted Files	Coverage Δ
app/dashboard/views.py	10.60% <0.00%> (ø)
app/dashboard/embed.py	31.60% <0.00%> (+3.44%)	⬆️

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