feat: Add admin billing paypal email

[iamareebjamal]

For fossasia/open-event-frontend#5127

[codecov]

Codecov Report

Merging #7286 into development will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #7286      +/-   ##
===============================================
+ Coverage        63.92%   63.94%   +0.02%     
===============================================
  Files              259      259              
  Lines            13030    13038       +8     
===============================================
+ Hits              8329     8337       +8     
  Misses            4701     4701              

Impacted Files	Coverage Δ
app/api/schema/settings.py	100.00% <100.00%> (ø)
app/models/setting.py	94.35% <100.00%> (+0.13%)	⬆️
app/models/event_invoice.py	93.68% <0.00%> (+0.20%)	⬆️

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