Toggle metadata panel on up/down keypress

Previously 'up' brought the panel up, and 'down' brought it down,
which might conflict with expectations on scrolling. Up/down keys
now move the metadata panel to the opposite state, no matter what
the current state was.

Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/697
Change-Id: I53079d81042afb86354bf44e9dfd662adf1576cb
Merge "Toggle metadata panel on up/down keypress"