Code Insights: Add global (site) scope for the dashboards and insights

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/22559

Background

This PR adds a new level of dashboard/insights sharing level - Global. Currently, this sharing level gives the ability to share dashboards and insights across all sg instance.

But adding insights and dashboards (editing site settings essentially) is available only for admins. Reading from these settings (viewing insights from the global (site) settings subject still available for non-admins users)

Before	After

UI changes

In this PR we have a new share level in visibility sections in the dashboard creation UI and in the code insights creation UI