Fix "live shell" hint reappearing when frame changed

[RobinDaugherty]

Fixes the work just merged in #490. The hint was reappearing when the user clicked a different frame.

This logic didn't belong inside of the REPL prototype anyway, so moving it out of there. Using CSS to hide it, we only need to add a body class at startup, or later, but only once.

[coveralls]

Pull Request Test Coverage Report for Build 346467872

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.838%

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Totals
Change from base Build 346436937:	0.0%
Covered Lines:	490
Relevant Lines:	506

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💛 - Coveralls