Add stageprompt js library

Adding perfromance platform's stageprompt, which enables us to collect user
journey data from pages tagged with data-journey attributes in the
markup.

This is a much simplified version of the code we discussed in https://github.com/alphagov/frontend/pull/307; see the discussion there for context. 

This will be used at first for transaction wrappers. See https://github.com/alphagov/transaction-wrappers/pull/19 to review the related changes.

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