Add an operation link to promotions to toggle their status

Toggling a promotion's status to enabled or disabled requires editing the promotion and finding the proper radio button in the right sidebar. For convenience, we should add a "Disable" and an "Enable" operation to promotions that use a simple confirmation form to toggle the status.

Note: disabled promotions currently append "(Disabled)" to the promotion name in the promotions list. I'd prefer to see this be a separate column or even use separate tables to differentiate enabled vs. disabled promotions, but this is good enough indication for now.