LP-478 - Sort recently published exhibits at beginning of index by default

[chrisrlc]

https://culibrary.atlassian.net/browse/LP-478

• Resets weight to 0 for exhibits that are newly published (does not change weight if exhibit was previously published, returned to draft, then published again, to prevent any strange gaming of the system)
  • Exhibits are secondarily sorted by published_at dates after weight
• Does not change order in "Unpublished exhibits" or "Your exhibits" lists (default weight is set to 50 in database).
• Order can be reset still in "Order exhibits" superadmin dashboard, e.g. https://exhibits-int.library.cornell.edu/exhibits/edit