Identify first time contributors

[OmarShehata]

This implements the feature described here #85.

I branched off of #118 because I needed the contributors URL fix from there. This just checks the CONTRIBUTORS.md in the repo/branch you're merging into. If the user's name isn't there, they are declared to be a first time external contributor and concierge will @ mention outreach team members. I didn't bother differentiating between internal/external contributors because if it's someone's first contribution they are 99% probably an external contributor.

To do when merged:

• Updated templates in repo's to enable the firstContribution message.

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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✨ ✨ ✨I'm the Cesium Concierge and I'm here to test things. ✨ ✨ ✨

[ggetz]

Thanks @OmarShehata! Also make sure to update the templates in the relevant repositories once this is deployed.

[OmarShehata]

Thanks for the super speed review @ggetz ! And you were right, it's a lot simpler just listing outreach users in the template itself as opposed to adding a config. I added a test to make sure it doesn't post if a user is not a first time contributor, fixed that missing period in the template, and added a TODO bullet point in this PR to remember to update the templates in the relevant repositories.

[ggetz]

This is good to go. Let's check #126 works before deploying this change as well. And, of course, update the relevant repos when you deploy as well!

[cesium-concierge]

Thanks again for your contribution @OmarShehata!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

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✨ ✨ ✨I'm the Cesium Concierge and I'm here to test things. ✨ ✨ ✨

[OmarShehata]

Once deployed, this should start working right away, since the Cesium repo uses the default issueClosedTemplate which will tag me and Sarah. We can change that here or in the Cesium repo if we want to change who to tag.