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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[sonarcloud[bot]]

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