
Description:

Use the 'jmap_preview_annot' option to enable/disable
the IMAP PREVIEW capability and the FETCH PREVIEW attribute.

Config changes:

The IMAP PREVIEW extension is now enabled ONLY if the 'jmap_preview_annot'
option is non-NULL and begins with the '/shared/' prefix.


Upgrade instructions:

Note that existing installations that DID NOT set the 'jmap_preview_annot'
option will no longer advertise the IMAP PREVIEW capability or accept the
FETCH PREVIEW attribute.  Previously, if not set, FETCH PREVIEW would return
NIL.


GitHub issue:

Fixes issue #4665
