OO-2636: enhance search in coaching tool with different views if a single user is found or multiple, implement excel export with 2 sheets

OO-2830 fix several bugs in members display code, bring back configurable user properties context, also for avatar view

OO-2636: database setup, overview list of participants with statistics and custom attendance rate editor

OO-2515: save the master file untouched, deliver  more pixels for the print view of the members course element, add REST api methods to check if a particular size of portrait is available

OO-2429: move the foreign key from identity to user, remove double mapping of identity, add more unit tests for invitation, unify the methods to create the user and identity