deposit: conferences id value synchronization

* Syncronizes value of the hidden conference_id field with the id
  from conferences field.

* Adds SychronizedField class which can be reused to do this.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
deposit: conferences warning

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>