Code insights: Add dashboard creation UI

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/22228

Background

Since code insight is going to have a dashboard system for sharing and viewing insights in a more convenient way we have to provide some UI/creation UI that a user be able to create their own dashboard. This PR adds a simple form/creation UI for the new dashboard entity.

How to test

1. Go to /insights/add-dashboard
2. Fill out the form and click create a dashboard
3. You should be redirected to the newly created dashboard page

What have been done

• Create dashboard area routes
• Craft creation dashboard page layout
• Add setting change-logic
• Changing file structure according to new dashboard routes