`INPUT_MARGIN_TOP` is used on https://github.com/fcampo/gaia/blob/f60cf697ae9d60b9bdd2c53bf6c03d6f562fec57/apps/sms/js/thread_ui.js#L967 to recalculate the margin for the input now that we have a subject on its top.
I decided to create a `INPUT_MARGIN_BOTTOM` even if we don't use it anywhere else just for readability, but I can get rid of it anyway.