Updated the project_sidebar_refactor flag

What does this MR do?

1. Renames the flag to sidebar_refactor to reflect the wider scope of the changes covered by the flag (it won't be only about the projects)
2. Scoped the flag to the users to have granular control over the rollout strategy
3. Fixes the issue introduced in !58638 (merged) where the flyout menu wasn't shown for the "Requirements" item at all.

Testing

In the Rails console (rails c), enable the sidebar_refactor feature flag for your current user. The Requirements menu in the lefthand navigation should now behave exactly as "merge Requests"

Screenshots (strongly suggested)

	Without the flag	With the flag enabled for the current user
Expanded menu (:hover state)
Collapsed menu (:hover state)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the change is behind the feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

References #323178 (closed)