Create new attributes for synchronization information about group

- create new module for existing attribute synchronizationEnabled,
  testing value (true, false or null)
- create new module for new attribute lastSynchronizationState, no check
  needed, only information about attribute
- create new module for new attribute lastSynchronizationTimestamp,
  check if timestamp is in correct format
- use new attributes in GroupSynchronizerThread in GroupsManagerBlImpl,
  method run, try to save information about synchronization of this
  group
- create new method saveInformationAboutGroupSynchronization in
  GroupsManagerBlImpl running in the new transaction (use in
  GroupSynchronizerThread)

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IMPORTANT: Need to do after merging this commit

1] create two new attributeDefinitions
 - urn_perun_group_attribute_def_def_lastSynchronizationState
 - urn_perun_group_attribute_def_def_lastSynchronizationTimestamp
2] set rights for these two attributeDefinitions
3] add these information to the changeLog