forms: Allow the delegate field to keep its current value

forms: Allow the delegate field to keep its current value

When a patch is delegated at parse time (either through the
X-Patchwork-Hint mail header or through delegation rules), the delegate
might not be in the list of project maintainers.

Add the current delegate to the list of acceptable values for the
delegate field to allow the current value to be kept when editing the
patch.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>