Make Sendgrid retries configurable

[c-w]

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[codecov-io]

Codecov Report

Merging #259 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   98.77%   98.77%   +<.01%     
==========================================
  Files          14       14              
  Lines        1141     1143       +2     
==========================================
+ Hits         1127     1129       +2     
  Misses         14       14

Impacted Files	Coverage Δ
opwen_email_server/config.py	100% <100%> (ø)	⬆️
opwen_email_server/services/sendgrid.py	100% <100%> (ø)	⬆️

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