Unified importer, no role change and able to send goodbye emails

No more need to deal with 2 plugins, since in this release, the acc-periodic-sync functionality has been integrated into the acc_user_importer. Here is a list of improvements:

• the plugin now has an integrated Cron timer to periodically import membership
• the plugin no longer modifies the user role
• new setting to decide how to set the username when creating a new user (firstname lastname, imis_id, contact_id)
• new setting to update or not the username after creation
• fixed major bug where only 100 first users were updated
• improve membership expiry detection
• fix sending of goodbye email whenever membership expiry date happens
• some hooks have been added