Migration to add many missing db indexes.

I noticed when doing some promotions testing that `code` in the `spree_promotions` table was not indexed. This causes a full table scan in a number of circumstances related to promotions. JDutil noticed this as well at some point, and added a bunch of indexes. Now I'm stealing his work, and taking some credit for it!
@athal7 Sorry about that. I must have tested this against the wrong DB.  This portion of the migration is now removed.