Gate new dashboard creation with customizable_dashboards feature flag

What does this MR do and why?

• Introduce :wip feature flag customizable_dashboards
• Hide New Dashboard creation UI behind the flag
  • New Dashboard button
  • Action dropdowns
  • Page description
  • Creation banner
  • Update description
• Disable /new dashboard route
  • I've allso fixed an issue where the route was still reachable if typed manually for groups

Notes for the reviewers

• After approval, before merging, I'm going to squash commits and add a changelog entry (assuming we need it).
• We are using this branch as a feature branch, so that we can merge all changes in one go and disable the flag. ( discussed here !185377 (comment 2410220200) )
• Keeping the MR in Draft to make sure it does not get merged accidentally before the above points.

References

• Disable New Dashboard creation (#525959 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Make sure that you're on the GitLab Ultimate plan
• Visit the group or project analytics dashboard e.g. http://gdk.test:3443/gitlab-org/gitlab-shell/-/analytics/dashboards
• Notice how the dashboard creation UI is hidden
• Enable the flag echo "Feature.enable(:customizable_dashboards)" | gdk rails console
• Notice how the dashboard creation UI is shown

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.