Add and remove collaborators for project

[taylordowns2000]

Note that we use the term "collaborator" for the model project_user which is a junction table (many to many) between project and user.

This feature will enable project collaborators with roles [:admins and :owners] to be able to add ore remove collaborators in a project and assign roles to them.

There are two scenarios for adding a user to a project – new users and existing users.

Todos

• [x] #1836
• [x] #1837
• [ ] #1835

Related to this:

• [x] #753

[christad92]

Moved this back backlog as #1835 is not currently a priority for the short term goal - April 30.