Fix grey block flashing on conversation window open

From the CallView commentary:

All the show/hide logic is done using the display CSS attribute on the child .media-display-area div.  This allows $el's display to be used purely to express layout (i.e. "table-row", as of this writing).  This is all motivated because it's a way to avoid a race between initial markup layout and JavaScript manipulation of the DOM.

Fixes <https://trello.com/c/1MZlQ0hy/>