admin-analytics: replace sidebar "statistics" with new "analytics"

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/39966.

This PR:

• Moves "Feedback survey" site admin sidebar to "analytics" sidebar group
• Replaces old "Statistics" sidebar group with new "Analytics sidebar group under
  • It can be disabled by setting the admin-analytics-disabled feature flag
• Also removes user-management-disabled feature flag key and uses existing admin-analytics-disabled for new user management disabling

Test plan

• sg start
• Open http://localhost:3080/site-admin
• Check that it opens a new "analytics/overview" page and there is no "Statistics" sidebar item
  • Also check that http://localhost:3080/site-admin/users opens new user management
• Open http://localhost:3080/site-admin?feature-flag-key=admin-analytics-disabled&feature-flag-value=true
• Check that it opens the old "statistics/overview" page and there is no new "Analytics" sidebar item
  • Also check that http://localhost:3080/site-admin/users opens old user management

Screenshots

Before	After

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.