Add conditional polyfill for promise, fetch

[aduth]

Fixes #1397
Fixes #1863

This pull request seeks to polyfill Promise and Fetch for environments where they are unavailable (IE11). It uses document.write ( 😱 ) to inline script dependencies for browsers which do not meet the necessary conditions, in a yepnope fashion. I sourced the technique from another project I maintain (source).

Testing instructions:

Verify that there are no network requests (in browser developer tools Network tab) for fetch or promise polyfills in capable browsers.

Verify that the Gutenberg demo content loads in IE11 without errors.

[afercia]

Good news: I can access the Demo page with IE 11
Not so good news: I get random (and obscure) errors and also managed to crash IE 11. Refreshing the page multiple times, errors happen inconsistently which makes me think at loading order of the scripts or something like that.

[afercia]

See IE 11 errors in the comments.

[aduth]

I'm unable to encounter any such errors.

Wonder if this could be related to #2066.

In any case, I don't think they'd be related to the polyfills (if polyfills failed to load for any reason, I expect the editor wouldn't show at all). Let's merge this and try to narrow down root cause of other errors in a separate issue.

[codecov]

Codecov Report

Merging #2041 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2041   +/-   ##
=======================================
  Coverage   20.24%   20.24%           
=======================================
  Files         130      130           
  Lines        4184     4184           
  Branches      712      712           
=======================================
  Hits          847      847           
  Misses       2811     2811           
  Partials      526      526

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