Implement basic sponsor management

[McPringle]

A sponsor should have the following properties:

• id (unique)
• version
• name
• description
• logo (only upload, use id as filename)
• link
• level (silver, gold, platinum)
• status (inactive, active)

The RESTful JSON API:

• POST /api/sponsors: Create a new sponsor (secured, admin only)
• GET /api/sponsors: Get a list of all sponsors
• GET /api/sponsors?level={level}: Get a list of all sponsors with the specified level
• GET /api/sponsors/{id}: Get the sponsor with the specified id
• PUT /api/sponsors/{id}: Update the sponsor with the specified id