Add setting to globally disable asking for a copy of answers

[stephencdaly]

What problem does this pull request solve?

Trello card: https://trello.com/c/h4cqFw0X/3084-release-copy-of-form-fillers-answers-feature

There is a setting on the form to configure whether or not the user is asked if they want to ask for a copy of their answers by logging in with One Login.

Add a global setting to allow us to disable this feature for all forms in the case that One Login is unavailable.

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