WIP: User can create organisations

Dev steps:

• Add "super_admin" column (boolean) to user
• Create organisation and membership model (with validations)
• Organisation must have a name
• Membership must have a user and organisation
• Membership has boolean column is_admin
• When new organisation is created, the created user becomes an admin of that organisation
• User can create organisation
• Super Admin can access and modify info on all organisations
• Integration tests