DevTools: Don't show "context" for classes without either contextType or contextTypes

[bvaughn]

• Don't show "context" for classes without either contextType or contextTypes
• For classes with contextTypes, change the label to "legacy context" instead

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Originally reported by @gaearon via bvaughn/react-devtools-experimental#368

[hristo-kanchev]

I'm working on this one.

Just clarifying if anyone else is looking at this atm.

[hristo-kanchev]

@bvaughn I've fixed the issue. Once we merge the DevTools in master, I'll create the PR. 🚀

[bvaughn]

Thanks @hristo-kanchev. I'll do my best to get #16381 merged as soon as possible.