allow passing key to [stackexchange]

[chris48s]

closes #5415
closes #3591

This is the code bit. It allows us to optionally pass a key when calling the StackExchange API. Once deployed, I'll also need to set a stackapps_api_key in prod. I've tested locally with a key and can confirm that without auth our quota is 300 per day and passing a key raises our quota from 300 requests per day to 10,000. This should cover our current usage (although these have been unreliable for a long time, so I suspect that contributes to the low traffic on these badges).

If we ever outgrow that:

• I think there is a mechanism to request an increased quota if we need it
• We could look at increasing the cache lengths on these

[shields-ci]

	Warnings
⚠️	This PR modified the server but none of its tests. That's okay so long as it's refactoring existing code.
⚠️	This PR modified service code for stackexchange but not its test code. That's okay so long as it's refactoring existing code.

	Messages
📖	✨ Thanks for your contribution to Shields, @chris48s!
📖	Thanks for contributing to our documentation. We ❤️ our documentarians!

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[calebcartwright]

lgtm. couple inline thoughts but nothing that needs to block

[chris48s]

Modifying the docs has dismissed the review. If you can give me a quick re-:+1: on this one I'll sort out getting it deployed with a key over the weekend.