Update available visualizations selector

What does this MR do and why?

Update available visualizations selector for New Custom Dashboard Flow - Available visualiza... (#422025 - closed).

This includes the following changes:

• Update the available visualizations selector on custom dashboards.
• Add a "Add visualization" button to dashboard in edit mode.
• Remove the new dashboard empty state #422025 (comment 1545630385).
• Update validation to allow no panels when editing saved dashboards.

Note: Analytics dashboard is an experimental feature hidden behind a default off feature flag

Screenshots or screen recordings

Recording

Analytics-dashboards--Gitlab-Org-Snowplow2--GitLab__1_

Visualization selector

Before	After

New dashboard empty state

Before	After

Edit existing dashboard state

Before	After

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Enable custom dashboards by following the alert prompt on Project > Analytics > Analytics dashboards.
4. Create a new custom dashboard by clicking on New dashboard on Project > Analytics > Analytics dashboards.
5. Edit your newly created custom dashboard by clicking on Edit button, making some changes and clicking Save your dashboard.

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 #422025 (closed)