Refactor user creation logic for clarity

[jsharkey13]

Some functions had unhelpful names, and some operations were not done in the most sensible order. Separating out validation and storing should help when we come to split apart creating and updating.

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 34.85%. Comparing base (098a30c) to head (d3bb6a1).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
.../cl/dtg/segue/api/managers/UserAccountManager.java	37.50%	2 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
- Coverage   34.86%   34.85%   -0.01%     
==========================================
  Files         520      520              
  Lines       23327    23326       -1     
  Branches     2841     2841              
==========================================
- Hits         8132     8131       -1     
  Misses      14361    14361              
  Partials      834      834              

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