fix(subscriptions): preserve recipient separators on unsubscribe

[immanuwell]

Small bug in Annotations.Unsubscribe().

If an annotation has a few recipients, unsubscribing one of them rebuilds the value with no ;. So my-channel1;my-channel2;my-channel3 turns into my-channel1my-channel3. Kinda busted, and the remaining subscription stops matching real recipients.

This patch keeps the ; separator and adds a regression test.

Repro

1. Set notifications.argoproj.io/subscribe.my-trigger.slack: my-channel1;my-channel2;my-channel3
2. Call Unsubscribe("my-trigger", "slack", "my-channel2")
3. On master you get my-channel1my-channel3
4. With this patch you get my-channel1;my-channel3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.35%. Comparing base (da04400) to head (352e687).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
+ Coverage   55.41%   57.35%   +1.93%     
==========================================
  Files          46       49       +3     
  Lines        4125     3782     -343     
==========================================
- Hits         2286     2169     -117     
+ Misses       1511     1254     -257     
- Partials      328      359      +31     

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