[Red Box] Umbrella issue for remaining polish

[Timer]

This is an umbrella issue for remaining Red Box issues. Please let me know if I forgot any.
PR: #1101

Blockers

• Clicking arrows currently makes their text select. Are those implemented as buttons? We need to make sure clicking doesn't select text.
• We also need to make sure this dialog is accessible. Can you please read accessibility guidelines for dialogs and make sure we follow as much as possible? This might mean specifying some ARIA attributes etc.
• Good point, this should also look reasonably well with mobile exultation.
• There is an annoying screen jump when switching between source / compiled because compiled includes a line with ^. In a perfect world I would like to see the line with | ^ get hidden, and instead see ^ rendered absolutely below the offending character. Yeah I know this is not fun. 😄
• Failing eject (only on CI?) Add runtime error overlay #1101 (comment)
• webpack/core patch Bump source-map-list to 0.1.7 webpack/core#30 (comment)
• babel-code-frame indentation Add runtime error overlay #1101 (comment)
• Double scrollbar—main content must never scroll horizontally Add runtime error overlay #1101 (comment)
• Message at the bottom Add runtime error overlay #1101 (comment)
• (Unrelated but I'd like to see this addressed) Add runtime error overlay #1101 (comment)
• Some design work on the syntax overlay screen to make them more visually cohesive. Not just in palette but in how they feel like. Syntax overlay is a lazy "terminal" output and wasn't really "designed". I'm not sure how to fix this and I don't want to block the PR on this but we need to make a conscious decision here.
• Remove use of environment variable to force color https://github.com/Timer/create-react-app/issues/11

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• Add documentation around Red Box https://github.com/Timer/create-react-app/issues/7
• Make the close hint work as a button https://github.com/Timer/create-react-app/issues/8
• Make the source <-> compiled toggle javascript-y and on per-stack-frame basis https://github.com/Timer/create-react-app/issues/10
• Make the internal calls expandable https://github.com/Timer/create-react-app/issues/9
• Fix source map bug https://github.com/Timer/create-react-app/issues/12 (Add runtime error overlay #1101 (comment))
• Reject hot reload on error
• Add method for switching between errors
• cursor: pointer all the (clicky) things!
• Hide arrows when only one error
• Remove whitespace (do in babel-code-frame)
• Fix text for expand to change omitted text
• Only code should scroll horizontally; not the page
• Ensure bootstrap hidden from output
• Add "This screen is visible only in development. It will not appear when the app crashes in production. Open your browser’s developer console to further inspect this error."
• Add runtime error overlay #1101 (comment)
• Add runtime error overlay #1101 (comment)
• change omitted to collapsed and expanded
• add collapse/expand to top as well

[gaearon]

It's okay now. 😄
We can do more later in a separate pass.