Replaces internal usage of utilities with hsds4 utils packages (plus Helix)

[tinkertrain]

Feature

As a sort of a proof of concept to make sure the newly released utils from HSDS 4 work fine, in this PR we:

• Update React to 16.14, which is the minimum version that supports the automatic runtime that React 17 uses (and we use in HSDS 4)
• Use all of the newly released @hsds/utils-* in place, the exported utilities remain like that
• Remove @helpscout/react-utils
• Remove @helpscout/helix adn use @hsds/helix instead

In theory, nothing changes or breaks, and that is the point of this PR, we don't even have to release it.

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