Allow admins to disable "Product research and feedback" for users on their instance

[quinnkeast]

Part of implementing RFC 215: Improving User Feedback.

Description

Based on @dadlerj's feedback, allow site admins to disable (enabled by default) the "Product research and feedback" page for users on their instance.

I imagine this would be a JSON config option at the global level.

Other implications

This will affect how confirmation content appears in #17645.

[github-actions]

Heads up @Joelkw @felixfbecker @jeanduplessis - the "team/web" label was applied to this issue.