Adapt login of participants for 2018

This includes the following changes

• Parse a backend flag to have the registration for sponsors closed / opened
• Automatically select the correct view on /join-team if there already is an invitation
• Change translations files for 2018 to remove references to old donation partner and old year
• Change available tshirt sizes to S, M, L, XL
• Remove outdated and broken /sponsoring page
• Change payment details to current payment details
• Add option to display stacktrace on 500 page
• Add npm run dev command to run server with nodemon