Refurbish events API

The events API needs to be redone for a few reasons:

1. Incomplete validation and error handling
2. Difficult to implement permissions - redoing this would make #15 (closed) much easier
3. Many redundancies and inefficiencies
4. Doesn't make use of the builtin features in django-rest-framework

One possible difficulty is that some changes might require corresponding client changes, so we might run into yet another case where we have another server-client incompatibility.