Add backrefs

[jenterkin]

We need backrefs from Event, and classes that extend from Events, to User so we can reference all associated Events with a user. For example:

    user.events
    user.work_events

Then update all places where we are querying for this information. This will make our code easier to read and to write (no more querying for something so common).
For more information see first comment here.

Edit: On second thought, let's not create a backref from Event, only the classes that extend from it.