Add analytics dashboard editor feature flag

What does this MR do and why?

This MR hides the dashboard & visualization create and edit workflows behind a new feature flag for #406814 (closed).

The following changes are made:

• Add a new project development feature flag combined_analytics_dashboards_editor
• Update dashboard list to hide new dashboard and visualization editor buttons.
• Update custom dashboard view to hide the edit button.

Related issues.

• Document flag Update product analytics development feature flags (gitlab-com/www-gitlab-com!124541 - closed)
• Rollout [Feature flag] Rollout of `combined_analytics_d... (#411407 - closed)

There is a follow-up to update the group's docs page.

Screenshots or screen recordings

Before	After (disabled)	After (enabled)

How to set up and validate locally

Note: This feature has a lot of setup steps. If you need help please ask me or I can step through these during a call with you.

1. Follow these instructions to setup Product Analytics in GDK and configure a project.
2. Configure a custom dashboard project like in !112481 (merged).
   • This needs a repository with a main branch in the same group as your test project.
3. View the dashboard list at Project -> Analytics -> Dashboards.
4. Verify that the new dashboard and visualization editor buttons are not rendered.
5. Enable the editor flag by running the command: echo "Feature.enable(:combined_analytics_dashboards_editor)" | rails c
6. Verify that the new dashboard and visualization editor buttons are rendered.
7. Create a new custom dashboard by clicking on the new dashboard button in the list view.
8. Verify that you can edit the custom dashboard by clicking on the edit button in the dashboard view.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #406814 (closed)