Stage Rails 5.X upgrade

Main tasks are:

• update crabgrass-media
• get rid of prototype-rails (use compatible prototype-rails branch)
• make tests pass with rails 5.1
• get rid of deprecation warnings for rails 5.2
• make tests pass with rails 5.2
• deploy on staging server
• make file upload 'work' as in production
• initial tests by ourselves
• get logs to work on staging
• invite external people to test
• check configuration parameters changed in rails 5.2