Define registryShape and use it as a prop type

[anlesk]

Reasons for making this change

With #699 being merged to master, registry became a mandatory parameter for most widgets and fields.
registryShape is being defined and used as a custom shape in components.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
  • If I'm adding a new section, I've updated the Table of Content
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[anlesk]

Well, I've run cs-format locally and it seems that the pre-commit task overrides some changes.

[anlesk]

It appears that running precommit scripts on node 8x leads to unexpected precommit code changes.
I was able to successfully commit with node 6x.

[edi9999]

This MR looks goot do me.

[epicfaace]

This was implemented in #1118 (see https://github.com/mozilla-services/react-jsonschema-form/pull/1118/files#diff-3a699a0719518f967e9b2df3f5549e02R3), so closing your PR. Sorry this PR wasn't merged in time.