Make synchronization service extendable, update userprojects model

what happens if an assignment is deleted and then later added again? because then the edb contains the info, that assignment of user x to project y (with uuid Assingxy) is deleted and will probably not show up as results in queries.

also the naming schema isn't unique.

what if there are 2 projects and 2 users?

project1: project1
project2: roject1
user1: user
user2: userp

now if you add user1 to project1 and user2 to project2 in both cases the uuid is the same "Assignuserproject1"