insights: [FE] Implement gql API for dashboard creation/edit UI

Created by: vovakulikov

Context

Things to implement on these pages:

1. getInsightsSubjects - this method provides a list of possible dashboard visibility things. (at the moment all dashboards are stored in setting cascade and therefore at the moment we use setting subject there)
2. createDashboard mutation
3. updateDashboard mutation

Note that if you have to extend something in the API that should be supported in setting-based API too.