Merch admin page facelift

The merch admin page, long hated by the Ampwall team, has finally gotten an update. It now looks similar to the album admin page with visibility controls and images on the left, and a series of dropdown menus on the right broken up by section.

The behavior is largely the same, a few UX improvements and wording updates but nothing should be too shocking. Relevant docs have been updated with new screenshots and text as well.