Add secret to helm chart

[james-otten]

Add SLACK_ADMIN_NOTIFICATIONS_WEBHOOK_URL secret to helm chart.

• [ ] Add the value to both environments in the GitHub UI

[codecov[bot]]

Codecov Report

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

Project coverage is 95.18%. Comparing base (3b92e69) to head (b062921).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## andrew/phone-deduplication #455 +/- ## =========================================================== Coverage 95.18% 95.18% =========================================================== Files 109 109 Lines 5480 5480 =========================================================== Hits 5216 5216 Misses 264 264 ```

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