Provide SC with list of what image groups have been requested by them

[stsccfr]

As a spin-off of #1535, Jackie requested that when an SC requests an image group to be allocated by the CC, they have some record of that. She and I discussed 3 possibilities:

1. when the CC is sent the email requesting the allocation, the SC should also get a copy of that request
2. add a Status column to the table "Image Groups Allocated to Syndicate as defined in app/views/manage_syndicates/image_server_group_by_syndicate.html.erb. The Status should be "Requested" or "Allocated" or "Assigned" as appropriate.
3. add another Action button to the same page as in (2) labeled "List Requested Image Groups" to show a similar table of image groups that are currently awaiting the CC to allocate them to the requesting syndicate.

I presented these to Jackie and Doreen and they thought that (2) or (3) were acceptable, and Jackie made the point that (3) makes sense only for SCs who make many requests of image groups from other counties. I can't see that happening very often, so (2) seems like the best one to implement.