Add vars for "Add install-create event handlers"

[james-otten]

Add the env vars:

• SLACK_JOIN_REQUESTS_CHANNEL_WEBHOOK_URL
• OSTICKET_API_TOKEN

OSTICKET_NEW_TICKET_ENDPOINT is omitted, let me know if you can't live without it.

The actual values, which I do not have, need to be added to the repo for CI

[codecov[bot]]

Codecov Report

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

Project coverage is 92.41%. Comparing base (332c7e5) to head (2ceae2d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## andrew/install-create-events #617 +/- ## ============================================================= Coverage 92.41% 92.41% ============================================================= Files 78 78 Lines 3176 3176 ============================================================= Hits 2935 2935 Misses 241 241 ```

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