Flask: refactor email functionality into Mailer class

[kevinlul]

Requires #1153 and has that merged in already

This is not urgently needed functionality and can be looked at another time.

[codecov-commenter]

Codecov Report

Merging #1155 (5904791) into master (d02010b) will increase coverage by 0.09%.
The diff coverage is 68.77%.

@@            Coverage Diff             @@
##           master    #1155      +/-   ##
==========================================
+ Coverage   69.24%   69.34%   +0.09%     
==========================================
  Files          25       26       +1     
  Lines        3239     3256      +17     
==========================================
+ Hits         2243     2258      +15     
- Misses        996      998       +2     

Flag	Coverage Δ
pytest	69.34% <68.77%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask/app/madmin.py	97.56% <ø> (-0.03%)	⬇️
flask/app/schemas.py	100.00% <ø> (ø)
flask/app/email.py	25.86% <20.37%> (-6.06%)	⬇️
flask/app/manage.py	19.84% <40.00%> (-0.95%)	⬇️
flask/app/blueprints/misc.py	69.93% <50.00%> (ø)
flask/app/login.py	56.41% <56.41%> (ø)
flask/app/stager.py	88.37% <88.37%> (ø)
flask/app/__init__.py	55.88% <100.00%> (-15.55%)	⬇️
flask/app/blueprints/__init__.py	100.00% <100.00%> (ø)
flask/app/blueprints/analyses.py	71.42% <100.00%> (ø)
... and 15 more

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[kevinlul]

Now fixed, though this specific use case is no longer needed with exome automation.

[frewmack]

Looks good to me!