Decouple people block plugin Model html generation
As a follow up for !803 (merged). This applies the same refactor to PeopleBlockPlugin removing HTML generation from models.
Here is a build: https://travis-ci.org/rafamanzo/noosfero/builds/117720302
As it involved many changes, I figured it would be worth of its own MR. I'll work on other block plugins refactor following. Please let me know if you do not want one MR for each.