Add customer prefix in url of customer resources

[EliuX]

We need to add the customer id in the URLs of the resources that belong to a particular customer, e.g.

• customers/{customer-id}/projects
• customers/{customer-id}/project-types
  TODO:

• Apply this change to all endpoints of projects
• Apply this change to all endpoints of projects types
• Correct it in the corresponding tests

[EliuX]

After discussing about it with @ralden, @enriquezrene and @Angeluz-07 and we came up to the conclusion that the customer_id are optional fields and the relationship between Customers and projects and project types is meant for grouping and not for ownership (You should not GET a 403 for not fulfilling this condition). So instead of enforcing this relationship, we are going to enforce that the time entry just access the items he truly owns. For that a new ticket will be created.