UID can be changed from the API

Description

A user's UID can be changed via the API. This is not allowed in the GUI, and the API should reflect it.

Distribution Name and Version

Whatever is on demo.fusiondirectory.org

FusionDirectory Version

1.2

PHP version used

Whatever is on demo.fusiondirectory.org

Origin of php packages

Whatever is on demo.fusiondirectory.org

Steps to Reproduce

1. use setfields() method in the API to set field 'uid'
2. Observe user now has a new UID.

Expected behavior:

UID should not be changed, and FD should warn the user that changing the field is not allowed.

Actual behavior:

The UID of a user can be changed without warning (or error).

Reproduces how often:

100%

Additional Information

Example of JSON changing the uid of user uid=newuid,ou=people,dc=fusiondirectory,dc=org from 'newuid' (back) to 'user':

DEBUG:root:Posting this JSON:

{'params': ['aucqmpj43inps9e7e8elljc6r0', 'USER', 'uid=newuid,ou=people,dc=fusiondirectory,dc=org', 
  
  {'reference': 
    {'refs': None}, 
    'userRoles': {'groupsMembership': ['cn=ogroup-test,ou=groups,dc=fusiondirectory,dc=org']}, 
    'user': {'uid': 'user', 'base': 'dc=fusiondirectory,dc=org', 'givenName': 'antoine', 'description': 'Some description', 'cn': 'antoine foobar', 'sn': 'foobar', 'userPassword': '{SSHA}+fxT2utzO5JGkfgeHxIxdqQE3hTsUjtN'},
    'ldapDump': 
      {'dump': None}, 
    'personalInfo': 
      {'fdPhotoVisible': True, 'fdPrivateMail': ['private1@se.se', 'private2@se.se', 'private3@se.se', 'private4@se.se']}, 
    'posixAccount': {'gecos': 'antoine foobar', 'groupMembership': ['cn=posixgroup-test,ou=groups,dc=fusiondirectory,dc=org'], 'force_ids': False, 'gidNumber': 1103, 'homeDirectory': '/hjhj', 'loginShell': '/bin/ash', 'posixStatus': 'active', 'mustchangepassword': False, 'shadowLastChange': 17625, 'uidNumber': 1102, 'primaryGroup': 1103}
  }], 'id': '123', 'method': 'setfields'
}