Add registration endpoint

[c-w]

Resolves #64

Resolves #75

[c-w]

@adamsclafani You may want to have a look at this change to the Celery configuration that I applied to make to run the pipeline end-to-end in Docker: https://github.com/ascoderu/opwen-cloudserver/pull/86/files#diff-4e42e46a683445dbbd13479c1d2ac43f. For some reason, the task routes now seems to require a string as key (fully qualified path to the task function).

[codecov-io]

Codecov Report

Merging #86 into master will increase coverage by 0.16%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   93.41%   93.58%   +0.16%     
==========================================
  Files          14       14              
  Lines         668      701      +33     
==========================================
+ Hits          624      656      +32     
- Misses         44       45       +1

Impacted Files	Coverage Δ
opwen_email_server/constants/events.py	100% <100%> (ø)	⬆️
opwen_email_server/config.py	94.73% <100%> (+0.98%)	⬆️
opwen_email_server/services/sendgrid.py	80.72% <100%> (ø)	⬆️
opwen_email_server/utils/log.py	87.71% <100%> (+0.44%)	⬆️
opwen_email_server/services/storage.py	97.85% <88.23%> (+0.11%)	⬆️
opwen_email_server/actions.py	96.15% <95%> (-0.17%)	⬇️

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