Pass event on speaker public view

[JGAntunes]

Speaker public view should send the event that the speaker attended.

Right now, the speaker model on the backend has an array of participations (which include years where he came and where he didn't).

The idea is that (assuming that he only participates fully in one event) we send this on the speaker public view. Something like:

speaker: {
  ...
  participation: 'id-of-the-event-he-attended'
  ...
}

This will allow us to better filter speakers on the web app as well as identifying the event e participated in.

The implications on this will be in the server views and in the client models for now. Afterwards we will have to find a way to present it on the speaker view.