Fetch dashboard configurations from the backend

Problem to solve

Currently the frontend uses baked-in visualization configurations as a temporary solution while we lack a backend source.

Once the backend MR Add additional fields to product analytics proc... (!103613 - merged) is merged this is no longer required.

Note: For the first internal preview of analytics we can keep the default configurations baked-in.

Open questions

• Should we completely remove the all the product analytics dashboard configurations? We could replace them with actual repository configurations by adding it to GDK or first just sharing the files.

Proposal

Update the frontend to fetch dashboard configurations for the repository using GraphQL added in !103613 (merged).

Implementation plan

1. Update dashboards_list.vue to use GraphQL to get the dashboards and remove the temporary imports.
2. Update analytics_dashboard.vue to use GraphQL to get the dashboards and remove the temporary imports.
3. Remove the temporary JSON configurations in gl_dashboards/.
4. Update specs
5. Existing dashboards/visualizations for jitsu-supported projects should be unaffected. (Use the product_analytics_snowplow_support feature flag)