[styled-components v4] Various small improvements to the libdefs

[omninonsense]

Changes:

1. The return value of StyleSheetManager.prototype.render
2. All styled components implicitly support style, className, and children, so reflect that.
   People can still override them if they need to (e.g. wanting to narrow children with children?: React.ChildrenArray<…>) using Props.

Note: One big "breaking change" is that strict props are no longer "respected" on components. So, it won't catch typos any longer. This now more aligns with flow's new "only error if there's a runtime error" paradigm (unless I am wrong and this does cause runtime errors, but extra props shouldn't cause runtime errors).

Making it a draft while I still test some of this against our production codebase at work, too. The tests seem to be happy and this seems to work on one of our smaller projects. So, hopefully won't stay a draft for too long!

Also, this should probably be merged after #3667.

[omninonsense]

Fixed the CI error. Other CI errors are because of older styled-component libraries.

[omninonsense]

Ok in our larger codebase there was some issues with style being defined on one component in a weird way, but that was the only problem. So I guess this is ready for review.

[AndrewSouthpaw]

LGTM. Travis errors seem to be related to older versions that are running their tests for some reason, so I'm going to merge this in. Thanks!