CSS: Mediaobjects in figures: Replace table-cell with flex layout

When several <mediaobject>s are within one <figure>, they remain in one single row and thus overflow the surrounding <figure> box.

There's not much sense to define that all figures must be in one row (table-cell). Make them wrap instead.

Note that this requires #383 (closed) / !226 (merged) to get merged to work as expected.

Fixes #437 (closed)