[server] Add user icon functionality

[Kuchteq]

• Modify the schema.sql file to include a nullable profile_picture BLOB field. This will contain the jpg data of a profile picture.
• Modify all paths where the user's profile picture would be visible to account for the new field. You will need to change the openapi.yml spec file and the associated handler implementations. Those paths include GET /dashboard and GET /groups/{id}. Reading up on how to save and retrieve blob data using sqlc is a good starting point.