closes #53
move dislike query
rename user_like_group table to user_dislike_group  -> closes #66
bugfixes in messagemodel

 use it for all get operations

+ correctionde  bug dans users.ts

Implementation des getAllMessages et compagnie

created and implemented simpleGroupsOfGroups in tool.ts to facilitate the implementation of other functions

Added 2 new function getAllQuestionsReceived and getAllAnswersReceived in messageModels that are not implemented yet

groups.ts -> getAllAnnouncements (args: *GroupSet*); two new functions : getAllAnnouncementsSent, getAllAnnouncementsReceived