Fix for #2774

Now if “Discounts Applied” is checked, it’ll LEFT JOIN the
wp_shopp_meta table and show promotions applied to the order.
Fixed #2733 Setting height to maxheight instead of auto to correct issues with non-square images.
Add new 'shopp_allowed_inputattrs' filter
Add autocomplete, placeholder, and required as allowed attributes for cart text inputs
Add placeholder as an allowed attribute for checkout textarea
Adding discount code to single order admin discount list.
Correct typo in debug statement.
Add shopp_order_item_image_id to allow for filtering specific image on order line items in admin.
Fix reference to priceline from cartitem.product.
Add request parameter to shopp_account_template filter.
Fixed #2733 Setting height to maxheight instead of auto to correct issues with non-square images.
Merge branch 'master' of code.shopp.net:core/shopp
Prevent unnecessary Product load_data queries

A product that has no images will cause the Collection load_data query to load no results leaving the product images property empty. Later during template rendering product.coverimage calls product.images that checks if the product images property is empty. If it is, it fires a query to load the images (now trying to load them all).

This change keeps track of which loads have been tried (whether results were found or not) and prevents querying again unnecessarily.
Fixed product.image and product.coverimage to properly handle data loading requests

When loading just a cover image, it is important that a load_data request for 'images' will override the cover images requests and load all the images for the product so that the Theme API will respond as the developer expects it to.

The 'coverimage' request is a special case because it is designed to only retrieve a single entry that goes into the same 'images' property as when loading multiple images. That would make the other images calls believe that all images available were already loaded.

We now detect keep track of what data has already been requested to prevent from re-querying when unnecessary.
Merge pull request #2752 from crunnells/iss2744

Fix for #2774
Fixed #2722 Removed dev db_version fixes
Added Purchased record to the shopp_download_forbidden filter hook
Fixed #2751 Corrected order id request detection
Ensure added image is loaded into product for tests
Add GA tracking information