[HIRE] Allow employers to create sub-accounts

[neue-dev]

• A backend route for creating subaccounts: gives their other colleagues access
• Only accessible via logged-in employers
• Requires the employer to specify an email, a level of access or role (from an enum)
• Create separate routes for removing subaccounts and changing their roles
• Only one master account can exist -> this is the account in charge of creating new subaccounts