R1:c8afc741fa7d

Add support for placeholders

Summary:
Support placeholder text for inputs. We currently don't use this because it requires JS and doesn't degrade (no JS means you have zero idea what the input is for if it isn't separately labeled) but there are some cases where intent is obvious from context (for example, the search input in the menu bar, which is fairly obvious on its own and will soon have a magnifying glass icon) and in such cases it's much prettier and saves a bunch of space over an explicit label.