Allow users from LDAP to be selected in user dropdowns

• Show an option at the bottom of the list "search other TU Delft users"
  • Show this option separated in some way
  • Always display this option regardless of the selectize search query
• Add endpoint to search for users by name
  • Allow only queries of at least 5 characters
• Add endpoint to create user
• Show modal where LDAP users can be searched
  • Show spinning icon while query is being performed
  • Add option to create selected user and fetch ID
  • Insert the new user into the original dropdown
• Allow this functionality only in the management panel
  • add permission check to controller
• Add limit of 50 users