fixing bug #5154
using UserNotificationBuilder to separate notification stuff from PDC subscription business
bug #5154 : incorrect sender for pdc notification
- taking the opportunity to fix a little problem around the footer of the mail sent by delayed notification mechanism
- refactoring the inheritance of created PDC user notification classes and renaming them in order to be homogeneous with the existing ones
bug #5107 :  Add a new parameter server.mixed to permit a site to be mixed with a secured authentication and a site navigation in clear. By default, this parameter is false and we don't recommend to setup a site in a mixed mode as it is a flaw in the security
fixing bug #5094
catching error which blocks indexer thread
adding some logs and adjusting some logs level
bug #5154 : incorrect sender for pdc notification
- fixing unit test failures
Feature Events/Agenda : centralized services
- using ICal4J API to handle event occurrences / import & export
- adding the event entity & repository structure
- adding a structure to develop entities and their repositories
- adding time unit conversion tools
- improving unit rounding conversion
- improving period tools
- adding a lot of stuff around useful Datable.java interface and their implementations