Network: Replace superagent with fetch for HTTP requests

[jasonpaulos]

This is a continuation of #661, opened by @PureBrent.

[algochoi]

LGTM 👍

[jasonpaulos]

Another benefit of this PR is that our browser bundle size will decrease, since cross-fetch appears to use less browser code than superagent.

Note that there is an opportunity to further decrease our browser bundle size, since most modern browsers have native fetch support, meaning cross-fetch's polyfill code is likely unnecessary.

Here are the numbers I got when locally compiling with webpack under different configurations:

• With superagent (develop before this PR is merged): 473 KiB
• With cross-fetch (this PR): 430 KiB
• With no browser fetch library (this PR + additional modification): 422 KiB
  • I tested this by just commenting out the cross-fetch import in src/client/urlTokenBaseHTTPClient.ts and compiling with webpack again -- this is ok for producing browser code, but it would fail for any node versions which don't have native fetch support.

To properly pursue the final bullet point, we'd need a way of conditionally importing a fetch library in node, but not in the browser. Since that involves more complexity for only a marginal benefit, I've opted not to do that in this PR.