Resolve "Making instance-wide data tools more accessible"

What does this MR do?

Adds instance statistics namespace.

Moves existing convdev index and cohorts controllers and views to new instance statistics namespace.

Adds icon in top nav for instance statistics namespace.

Adds application setting for visibility of instance statistics.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Instance statistics features are only visibile if you have access to the admin panel, they should be easily accessible and optionally open to all instance users.

Screenshots (if relevant)

Top bar (latest):

Nav:

ConvDev index:

Cohorts:

Settings:

'Only admins' setting as non admin:

'Only admins' setting as admin:

'All users' setting as non admin:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #41416 (closed) Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48507