Improve group related information

• Refactored status tags to a helper instead of separate files everywhere
• Refactored group statuses. There is now a single "relevant group status", which can be requested as a hash. This hash contains all information to properly display group status to the user, with explanation of the status and the next step for the group.
• Improved information presented to students, coaches and clients
• Change groups to index
• Change hiding panels to be independent of panels. It can now be used on any HTML element
• Add coach invitations to My Groups, showing only invitations from active course editions
• Add archived groups to the overview