Wrapper function to avoid polluting global namespace

[frnhr]

Wrapped the clientside template JS code in an anonymous function. This prevents clientside and ns vars from polluting the global namespace.

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[alexcjohnson]

Thanks @frnhr - good catch, thanks for the fix!

If you merge or rebase on the tip of dev the tests should pass. Then we'll just need:

• a test that these variables are indeed not in the global namespace. Maybe in test_clientside.py pick any test and add a line at the end like assert dash_duo.driver.execute_script("return !(('clientside' in window) || ('ns' in window))")
• a changelog entry

[frnhr]

PR ready for review.

[T4rk1n]

Looks good! 💃