Enable creation of global promos

Currently the only way to create a global promo is to first create it as a local promo, then remove it from the page where it was created, and then add it back in as a global promo.

The attached patch adds a global checkbox to two places. First, as part of the thick box displaying content types as part of the promo creation process. This mirrors the use of the global checkbox when adding an already existing promo to a page. Second, at the top of the node creation form. This second checkbox will be checked or unchecked based on what the user did with the checkbox in the thick box. This gives the user a second chance to make this choice. Note that this checkbox only appears when creating a new promo node, not when editing an existing one.

In addition to adding these new global checkboxes, the patch makes a slight change in the presentation of the global checkboxes in the thick box. When either creating or choosing a promo, the global checkbox in the thick box now goes at the top rather than at the bottom. The reason for this change is twofold. First, the user needs to check the checkbox before clicking a link, or they will miss it. So it makes sense for this to be the first thing they see since it is the first step they need to take. Second, if the list is long enough, a checkbox at the bottom will be out of sight making it likely that a user will forget about it. Putting the checkbox at the top protects against these possible problems.