Settings for the admin

[Dudrie]

🎫 Description

Add a system that lets the admin configure some settings with a UI and without the need to fiddle with configuration files.

Those settings might include (more complicated ones should maybe be post poned):

• Standard team size
• Can tutors excuse students or can only the admin do this?
• Mailing settings (can maybe be combined with Make mailing settings optional #364)
• Settings for column names for auto-mapping for importing CSV files.
• Do inactive students count for the overall criteria statuses and if so in which way? [Setting] Students which status is not active should be excluded from the criteria calculation. #189
• Allow enabling / disabling certain features (ie modules, criterias, ...) by the admin (see Add support for short tests #502 for one idea).