Editor: bump figure margin so figcaption is clickable

[martymcguire]

On Firefox on Linux (at least), I can add an image and see the prompt to add a caption, but clicking on the caption results in a click on the next paragraph, taking keyboard focus there instead.

[martymcguire]

example of overlap with figcaption click area when a line of text follows an inserted photo.

[martymcguire]

Found a nice trade-off by giving the figcaption a min-height when the image editor is activated:

.medium-insert-active .medium-insert-caption-placeholder {
  min-height: 1em;
}