Add checks to prevent creation/access to personal snippets when the new option is disabled

Problem

We want to prevent enterprise users from creating/accessing personal snippets when Allow personal snippets option is disabled.

The configuration for Allow personal snippets is stored in the database (see Add allow_personal_snippets field to group sett... (#555051 - closed)).

Proposal

Additional Notes

The ability to toggle this setting should only be available to group owners and should only impact enterprise users in the namespace.

When this setting is enabled, Enterprise Users that are part of the namespace would be allowed to create snippets in their personal namespace.

When this setting is disabled, Enterprise Users that are part of the namespace would NOT be allowed to create snippets in their personal namespace.

Similar implementation

Disable personal access tokens for enterprise u... (!148415 - merged)

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