Code Insights: [BE] update mutations over insights don't work if insight isn't included in a dashboard

Created by: vovakulikov

Steps to reproduce

1. Create Insight without that won't be included in any dashboards (omit dashboard field in create insight mutation)
2. Try to run update line chart mutation for newly created insight
3. See error about no insight found

This should work since we can modify insight in "All insights" dashboard. In this "All insights" dashboard we fetch all available insights. Some of them can be insights without that are not connected to any dashboard. Mutations for those insights don't work but it should since users can modify insights in all insights dashboards. For example, update filters or create other insights as a copy