simplify media manager panel height
The height of the panel was calculated in JavaScript, trying to use the
remaining space. With larger headers that lead to a relatively small
area. In addition the height calculation behaved weirdly on certain
resolutions (in some templates) resulting in a decreasing height on each
image interaction.
This patch simply sets the panel height 60% of the browser window using
the vh unit (which was not available when the media manager was
written).
To keep backward compatibility and not accidentally break a bunch of
templates, the height is still set from JavaScript but only once instead
of updating it on every resize operation.
A proper refactoring of the whole media manager code is still something
we need to tackle in the future, but this change should make it slightly
better.