Create join models for the event attendees

Create an event_facilitator model and add the relation in event.
Create an event_fundation model and add the relation in event.
Create an event_provider model and add the relation in event.
Use the new event invitation models in the UI

Delete the old associations between events and facilitators, fundations
and providers.
Use the new associations in the controllers and views.
Fix view and model relation issues
Improve the flow for attending an event in the UI

If the member is only facilitator it should attend immediately.
If the member has fundations or providers show a dialog to select the
roles in which he/she is attending.
Merge branch 'issue-35-asistencia-eventos' of github.com:carloshlopez/consonrisas into issue35
merge issue35 event invites