Restore function for syncing

[stakats]

Reconciliation unimplemented and other conflict resolution errors should trigger option to clear local db and pull from server or clear server and push from client.

[stakats]

Some version of the existing restore functionality should also be moved from the gear menu to the sync preference pane. Example:

Restore my library:

From this machine to the Zotero server [radio button]
From the Zotero server to this machine [radio button]
Restore [button]

Warning message displays:

"You are about to erase all of your Zotero server data and replace it with the Zotero library contained on this machine. Are you sure you want to proceed?"
or
"You are about to erase this machine's entire Zotero library and replace it with your Zotero server data. Are you sure you want to proceed?"

Former option triggers server clear and sync. Latter option triggers local table drop and sync.