magit-{,key-mode-popup-}apply-mailbox: new key binding

In `magit-mode-map' bind "J" instead of "j" to
`magit-key-mode-popup-apply-mailbox' (or `magit-apply-mailbox' if
`magit-rigid-key-binding' is non-nil).  In the apply-mailbox popup
keymap bind "J" accordingly.