Version Control for Snippets: Allow starring snippets (Iteration N)

Important: All the new functionality must be implemented behind the feature flag :version_snippets. Besides, it has to be scoped for projects. Therefore, instead of checking if the feature flag is enabled or not (Feature.enabled?(:version_snippets)), we will check if the feature flag is enabled for specific users (Feature.enabled?(:version_snippets, user)).

We have to add the logic to allow users to star/unstar snippet repositories.

Availability and Testing

Might be a candidate for a new E2E unless sufficiently covered by lower-level tests