Explicitly permit within-list information sharing during embargo

Explicitly permit within-list information sharing during embargo

Permitting sharing of embargoed fixes amongst predisclosure list
seemed to have appropriate consensus.

IMPLEMENTATION TASKS:
 * Send a notification to the existing predisclosure list members
   informing them that they have been subscribed to the new list.
   Notice should point them to the policy section on filtering
   by List-Id, and offer to unsubscribe them from both lists if
   they prefer.
 * Create the new mailing list, and
   - check that it can be emailed from outside
   - that messages are held for moderation and can be approved

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
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v2: Obfuscate -discuss@ list's full email address with <dot>
    and <span>.