Create central user management table

Motivations

• Fast and consistent search
• Unicity of userId/e-mail
• Pagination
• Source of truth
• Change user from one user backend to another
• Block conflict on user creation

Plan

1. DB table design
2. Sync with backends
3. Design new API methods IUser to provide snowflake and accountName
4. Use accountName in the places where it makes sense
5. Replace UserManager implementation with new table
6. Replace DatabaseBackend implementation to use new table then drop its original table