* Whitelist files that can be embedded through url-loader

Change the current catch-all loader to use file-loader instead of
url-loader, and exclude common image file extensons. Add another url-loader
for images, configured identically to the original catch-all loader.

Part of issue #1293.

* Update tests to reflect change in loader config

* Integration test fix

* More CI fixes

* Work around broken loader-utils behaviour in integration tests

* Revise the documentation around file loading

* Documentation tweaks

* Docs tweak

* Update test now that webpack/loader-utils has been updated

* Update react-scripts to depend on babel-loader@7.0.0-alpha.2

* Bump deps