Better rate-limiting implementation

[justin0022]

Current rate-limiting is static, relying on https://www.npmjs.com/package/bottleneck, instead of dynamically throttling depending on the X-Rate-Limit-Remaining header on the response.

Implementing proper rate-limiting will:

1. Improve performance, potentially dramatically.
2. Reduce chance of accidentally hitting the rate limit when making batch changes and causing an absolute headache because it becomes difficult to determine which requests succeeded and which requests failed (as happened recently to me when running https://github.com/ubccapico/restore-speedgrader-comments.