Load dashboard list and single dashboard from selected repository

What does this MR do and why?

1. Checks if for this project a custom dashboards project is set, if not showing an info alert box. Button links to the settings page
2. If custom dashboards project is set, it loads file listing from .gitlab/dashboards/ and adds based on that items to the list
3. Dashboard Detail loads the full file from .gitlab/dashboards/selected-dashboard-file.yml
4. Based on the dashboard it loads if added the single visualization files from .gitlab/dashboards/product_analytics/visualizations/

Screenshots or screen recordings

Alert if no custom dashboard project is set:

How to set up and validate locally

1. Make sure you are on GitLab Ultimate
2. You need to set up the Product Analytics DevKit if you haven't already done so. And follow the instructions on Connecting GDK to your devkit.
3. Access /-/product_analytics/dashboards/ on an product analytics activated project like [GDK_HOST]/gitlab-org/gitlab-shell/-/product_analytics/dashboards/.

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.