ENH Refactor Element to function component

[emteknetnz]

Issue #1164

I'm splitting the linked issue into two PRs

• Firstly I'll do this one that only refactors the existing class component to a function component
• Once this is merged, I'll do a second PR to do the GraphQL refactor

Note that I've removed the getDerivedStateFromError() / childRenderingError functionality because:

• I didn't find a replacement for it using functional components
• We don't use getDerivedStateFromError() in any other components, so I think for consistency we don't actually want it

[GuySartorelli]

I am reliably getting this error in the developer console whenever I expand an inline-editable block and then click "save" in the page (note: NOT when I click the ... menu and save the block from there)

Uncaught (in promise) Invariant Violation: Store reset while query was in flight (not completed in link chain)

Other than that it seems to work as expected locally. Haven't taken a proper look at the code yet but at a glance it looks okay.

[emteknetnz]

That's an existing console error unrelated to this PR. It's coming from Apollo.

[GuySartorelli]

I had originally requested replacing getDerivedStateFromError() but it looks like error boundaries are no longer directly supported in react. While I think losing that functionality isn't desirable, keeping it is probably more effort than it's worth right now.

[GuySartorelli]

LGTM