Storage solution for user-created items with user-only-scope (no sharing)

[rubenthoms]

• Investigate database solution (see points in parent issue)
• Implement backend service
• Make frontend service for managing and communicating with backend (see https://github.com/equinor/webviz/blob/main/frontend/src/framework/UserCreatedItems.ts)
• Required for persistence of user-session