Create new HTTP endpoints for new event type-related features

[petterroea]

#43 changes how events work, introducing event types. When this issue is done, we need to add some new api endpoints so we can administer the new system.

Suggested endpoints:

• GET /event_type/:uuid - returns an event type (and all events associated with it). Open for all.
• GET /event_type - returns all event types(and associated events). Open for all
• POST /event_type - allows you to create a new event type. Only for admins.