Add link between user, devices and sessions

[mregni]

Investigate if we get a notification when a new user is added OR new devices is registered (user login)

If so, we can add a foreign key between sessions and Devices and sessions and Users.

Remark: What will we do when user is added when EmbyStat is offline, when will we get the new data? (Run small sync at server start?)

Linked to #17