Resolve "Offering should be multi-relation"

Closes #284 (closed)

• Added an offerer class that allows various types of offerers
• Added support for research groups and users to offer projects
• Modified the company show view to be used for all types of offerers
• Changed the project creation form to display only the contacts for the currently selected offerer
• Added an option for course editions to specify the offerer types that are allowed to offer projects
• Changed the project creation form to only show eligible offerers for the selected course edition (to be finished once the course edition selection is moved to another page)
• Split company projects and internships based on the offerer type (different panels for company projects, research group projects, personal projects)
• Many other changes to support the move from companies offering projects to various types of offerers offering projects