Merge branch issue#240.2 with master

Update to the purge process

• Do not send purge alert mails if the demand only contains hidden elements. (Issue #240 (closed))
• The hidden elements should appear in the admin area, in the DB demand, should be purged, but not appear in any email. (Issue #240 (closed))

Issue #240 (closed)

This commit already includes the changes from the following commit : bd50bd32