Specify course specific roles on project proposal

With #248 (closed), there should be a way to pass project roles along to the groups automatically based on the definition directly. This is mostly added in the original specification, but the option to mark some as required is not.

Implementation:

• Add to CourseSpecificRole: required: boolean (required to specify a person with this role on project creation)
• Add partial for specifying the roles of those involved in the project (= a cocoon), and add this to the project proposal page. The only options are the roles defined for the course edition. This partial is only shown if any course edition specific roles are defined.
• In project creation controller (and/or project model), add validation that all required roles are specified.

Questions:

• Should we allow configurations where multiples of a certain role are required?
  • Can be worked around using different names for these roles, i.e. Supervisor 1 and Supervisor 2 and marking both a s required. Won't have.