This includes reduced title font size to bring the total info bar
height to the same height as the buttons, and an image preview that
doesn’t hit the walls of the container.
Probably most properly we should check for any number of spaces that
isn't 4, but that's a bit more work to do with our linter framework,
and in practice basically every CSS whitespace error we see is 2-space.
This adds an event listener (by way of delegation) to the
.message_inline_image elements that pops up the overlay and hides it
when the overlay exit is clicked.
Fixes#654.