Event start flow

This is a rough (i.e. not final) pull request. Just wanted to run this by you before I went further. Here's what the code does so far:

(1) When event is started, generate actions for all participants. I'm not sure what actions we should generate, exactly, so for now I've just ported over the code from the rake task. I'm guessing the sorts of actions we create may need to be event specific?

(2) When the user joins a started event, we create actions for them. (Same set of actions are created as in (1)).

(3) When user completes an action, an action is potentially generated. This method does nothing for now.

Let me know what you think of this so far, @davidtlee! Once you give feedback, I will incorporate changes/add tests. Thanks!