[17.0][MIG] event_project

[rrebollo]

standard migration from v16 with several coding improvements:

• split business logic between event and project models
• sql constraint to avoid settle same project for different events (test added)
• refactor tests
• improve documentation
• replace custom project.task action by built-in
• add project search capabilities to event search view
• ensure groups event manager in views

[rrebollo]

@jelenapoblet Could you please do the functional review? I think the documentation could be improved. Maybe after testing it, you can come up with some specific suggestions.