Restructure UserAccounts

[rbergm]

An UserAccount should not be directly coupled with a Person. Instead the data actually necessary for an UserAccout should be copied from the person it is created for. Whenever the Person is updated the changes should be propagated to its corresponding UserAccount. This approach would provide the following benefits:

• Lower overhead upon login - no data from tables other the the UserAccount needs to be fetched. This should improve performance
• No need to make Person and the entities a Person references implement the Serializable interface. Implementing this interface causes a bunch of problems as for example the ParticipantProfile contains value objects such as the participant's date of birth which should not be serialized (see https://stackoverflow.com/questions/26451590/why-should-javas-value-based-classes-not-be-serialized)
• Cleaner code - an UserAccount actually is not interested in the whole Person with its ReferentProfile, ActivistProfile, etc. but rather only cares about the person's name, email and identifier.

[rbergm]

Solved by 3f2f768658bf5b21d785992d3c9d14d5a31ef59a