Contrib Key Sorting feature for a project's data sets

This branch follows the recommended revisions that have been proposed during our discussions on issue #1273

-Deleting a contrib key does not remove the ability to select data sets by that key until there are no more existing data sets that were uploaded with that key
-The keys are overlaid with letters that can be used to quickly identify them.  All data sets uploaded with "key1" will show an A, and data sets uploaded with "key2" a B, etc etc.
-Once no more data sets with a deleted contributor key exist, the existing data sets will be appropriately relettered.

Addresses issue #1273