offer a way to edit metadata when adding or editing cards+solution

If "*all+*add metadata" existed, then when any card foo was added, the card creation form would include a second section beneath the card editor and above the Submit/Cancel buttons, for which "*all+*add metadata" would be the form (much like +*content).

"+*edit metadata" would also be available, analogously to +*add help and +*edit help

This metadata section and its fields would be differentiable with CSS selectors, and probably have a different default look.