Add continuous export management api

To resolve INSIGHT-814, we need to add:

• The cron is there by default and send the event every week (that interval can be configured via nuxeo.conf (need a nuxeo reboot)
• We can activate the continuous export by setting a conf variable OR via REST and the UI
• We can start one export via the button in the UI even if the continuous export is disabled
• Each time Nuxeo restart, we are giving priority to the KVS to set the activation of the continuous export - meaning that if the conf variable is set to false in nuxeo.conf, priority will be given to the KVS entry if it has been set via Insight