Add "add-one" feature

[wopsononock]

Someone validates a tray and realizes there are more books to be added to it than were originally recorded. Allow this to happen without re-scanning entire tray.

[wopsononock]

Strategy is to map a button in the scan-end page to a Trays controller action that resets the tray status to "Incomplete." This should allow it to enter the initial scan workflow again where a new total # of items can be applied. Each new item can be scanned then.

[wopsononock]

There was some weirdness after trying to use this during the initial scan since it shares the scan-end page with the validation step. Testing during validation was successful. Test method was:

1. Scan new tray entering/scanning one less book.
2. Validator entering that number and scanning the books, ending up with one left over.
3. They click the add-one button and send it back to initial scanning, telling the scanner which book is missing.
4. The initial scanner enters the correct number of books and scans the missing one.
5. It goes back to validation with the accurate number and the tray is re-validated/submitted.

[wopsononock]

If the validator uncovers a miscount before beginning the scanning, they could be linked to the add-one feature from there if the validation count is more than the initial count. if it's less, the tray would need to be reinitialized.

[wopsononock]

This has been tested successfully