Framework: Preload common API data

[aduth]

Related: #1974

This pull request seeks to introduce a mechanism for bootstrapping common REST API requirements for use in with the withAPIData higher-order component (#1974). Specifically, this improves perceived performance by making endpoint data available immediately upon page load, avoiding components flashing in and out (e.g. LastRevision in ##1974, PostFormats in #2500) where the API data would otherwise load asynchronously.

Implementation notes:

While not entirely necessary for the changes here, I split the response between body and headers. This will be useful for future enhancements to the withAPIData higher-order component in allowing a "has more results?" type functionality.

Testing instructions:

Repeat testing instructions from #1974, noting that the LastRevision component appears immediately upon page load if revisions are available for a post. Ensure that there is no network request occurring to retrieve /wp/v2/posts/%d/revisions.

[codecov]

Codecov Report

Merging #2501 into master will increase coverage by 0.03%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #2501      +/-   ##
==========================================
+ Coverage   31.57%   31.61%   +0.03%     
==========================================
  Files         175      175              
  Lines        5304     5308       +4     
  Branches      914      914              
==========================================
+ Hits         1675     1678       +3     
- Misses       3081     3082       +1     
  Partials      548      548

Impacted Files	Coverage Δ
components/higher-order/with-api-data/index.js	82.66% <100%> (ø)	⬆️
components/higher-order/with-api-data/request.js	96% <87.5%> (-4%)	⬇️

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