Rate limiting

[sajidanower23]

What this PR does

Fixes #1283

Uses the express-rate-limit library to rate limit API requests to the gateway.

Known Issue

It does not use API Keys to rate-limit. To do that, get the API key from the request headers and resolve it to a user within the keyGenerator handler function of the limiter. I could not wrap that function inside a promise, so was not sure how I could use fetch to resolve an API key inside the keyGenerator function.

Checklist

• There are unit tests to verify my changes are correct Unit tests aren't applicable
• I've updated CHANGES.md with what I changed.
• I've linked this PR to an issue in ZenHub (core dev team only)

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