Moved mail helper classes into separate files

[runz0rd]

For #459

[SendGridDX]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #472 into master will increase coverage by 0.39%.
The diff coverage is 86.51%.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
+ Coverage   82.41%   82.81%   +0.39%     
==========================================
  Files           9       30      +21     
  Lines        1001     1024      +23     
  Branches      160      160              
==========================================
+ Hits          825      848      +23     
  Misses         86       86              
  Partials       90       90

Impacted Files	Coverage Δ
sendgrid/helpers/mail/__init__.py	100% <100%> (ø)	⬆️
sendgrid/helpers/mail/mail.py	97.51% <100%> (+9.24%)	⬆️
sendgrid/helpers/mail/asm.py	73.91% <73.91%> (ø)
sendgrid/helpers/mail/ganalytics.py	79.66% <79.66%> (ø)
sendgrid/helpers/mail/subscription_tracking.py	80.48% <80.48%> (ø)
sendgrid/helpers/mail/footer_settings.py	81.25% <81.25%> (ø)
sendgrid/helpers/mail/spam_check.py	81.25% <81.25%> (ø)
sendgrid/helpers/mail/category.py	81.81% <81.81%> (ø)
sendgrid/helpers/mail/click_tracking.py	82.6% <82.6%> (ø)
sendgrid/helpers/mail/content.py	82.6% <82.6%> (ø)
... and 34 more

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[mbernier]

awwwww yeah!

[thinkingserious]

Hello @runz0rd,

Thanks again for the PR!

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