Allow snippets to be disabled globally

Zendesk: https://gitlab.zendesk.com/agent/tickets/92929

Customer requests the ability to disable snippets globally for internal compliance/security reasons. Lack of history or audit-ability around snippets makes them a liability for the company.

In the related issue we took the stance that we didn't want to allow global disabling of features in order to encourage use of features like CI. I wonder if snippets can be an exception to this as snippets are not a core feature and can present an issue for certain customers as seen here.

@JobV What do you think? I fully understand the decision made in gitlab-ee#920 but am interested in your thoughts on snippets specifically.