Define an admin user flag

[allenporter]

Define an admin user flag that is given to the first user registered.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.37%. Comparing base (29e3392) to head (9253a97).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
supernote/server/services/user.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   70.28%   70.37%   +0.09%     
==========================================
  Files          62       62              
  Lines        6134     6141       +7     
==========================================
+ Hits         4311     4322      +11     
+ Misses       1823     1819       -4     

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