Add the ability to un-watch users via UnwatchUserAsync(), add WatchUserAsync() to replace the ambiguous AddUserAsync()

[jpdillingham]

A conversation with @mathiascode alerted me to the fact that I had omitted this capability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (301c60b) to head (6cad834).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #818      +/-   ##
==========================================
+ Coverage   99.39%   99.42%   +0.03%     
==========================================
  Files         183      184       +1     
  Lines        6257     6276      +19     
  Branches      947      950       +3     
==========================================
+ Hits         6219     6240      +21     
+ Misses         21       20       -1     
+ Partials       17       16       -1     

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[sonarcloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud