OO-1616: replace the view for the business group lists with several customized queries which load only the needed data

OO-952: move the "display members" configuration to the business group table, optimize the query for the roster (list of contacts)

OO-800: upgrade http client to the new 4.x serie, fix unit tests, remove deprecated multipart entity

OMA-87: enhance the REST API for notifications, add more informations about forum and folder notifications

OO-291: first step: move business gourp manager to a business group service, remove dependency to the context...