access_level: Custom permission for community users to access an article

- Added a field on article new/edit to specify which users can access the article
- Added a relationship has_and_belongs_to_many between Article and Person
- Created a new table article_allowed_users for has_and_belongs_to_many
- Added cucumber tests

(ActionItem2852)

Signed-off-by: Athos Ribeiro <athoscribeiro@gmail.com>
Signed-off-by: Andre Bedran <bedran.fleck@gmail.com>
Signed-off-by: Arthur Del Esposte <arthurmde@gmail.com>
Signed-off-by: Carlos Andre <carlos.andre.souza@msn.com>
Signed-off-by: Fabio Teixeira <fabio1079@gmail.com>
Signed-off-by: Gabriela Navarro <navarro1703@gmail.com>
Signed-off-by: Tales Martins <tales.martins@gmail.com>