Hide "live shell" hint after console has been used

[RobinDaugherty]

This is an implementation of the request in #315, but instead of removing the hint completely, it's shown only when the console has never been used.

The JS on the page sets a cookie when the user types in the console, and hides the hint if that cookie is present. (The cookie will expire after one year.)

Closes #342 Fixes #315

[coveralls]

Pull Request Test Coverage Report for Build 346125630

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-10.8%) to 85.532%

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Totals
Change from base Build 346059898:	-10.8%
Covered Lines:	402
Relevant Lines:	470

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 346125630

• 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 96.344%

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Totals
Change from base Build 346059898:	0.0%
Covered Lines:	448
Relevant Lines:	465

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💛 - Coveralls