Fix collapse.tsx for TS 2.4

[sandersn]

React.HTMLAttributes requires overflowY to be a member of a string union: "initial" | "inherit" | "unset" | "auto" | "hidden" | "scroll" | "visible" | undefined. In Collapse.render, the overflowY needs to be given the explicit type "visible" | undefined in order to be assignable to HTMLAttributes. Otherwise, it gets the type string, which is too general.

Fixes #1237

See also DefinitelyTyped/DefinitelyTyped#17581, which this change depends on.
See also discussion at microsoft/TypeScript#16047.

[palantirtech]

Thanks for your interest in palantir/blueprint, @sandersn! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[sandersn]

Note that this change can be merged before DefinitelyTyped/DefinitelyTyped#17581, but it won't fix the build error with Typescript 2.4 until both are merged.

[cmslewis]

Thanks for the contribution, @sandersn! My personal preference is to wait for DefinitelyTyped/DefinitelyTyped#17581 to merge. @themadcreator or @gscshoyru: quick thoughts?

[sandersn]

Actually, now that I think about it, it's probably better to merge this change first so that TS 2.3 users (and earlier) aren't broken when the React types change.

The reason is that with the current React types, the return type of Collapse.render isn't really checked in 2.3. When DefinitelyTyped/DefinitelyTyped#17581 goes in, overflowY will be checked better in both 2.3 and 2.4, and start failing.

[cmslewis]

@sandersn Gotcha, that makes sense. Lemme circle up with one other remote Blueprint dev to make sure we're in sync (the other locals are all out today).