Remove the synchronization button by default

Remove the synchronization button by default and add a documentation on Nuxeo Drive section for how to enable it (even better if it is in Studio).

Note: we could think of doing the same for Direct Edit for sake of consistency, but we actually do this for not encouraging for now the use of the synchronization, on the contrary we want people to use Direct Edit (it promotes Nuxeo Drive) so we will let it.