Clean up image diff 2-up mode

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Follow up after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14397 is done

Would love to have that same layout as in https://github.com/cameronmcefee/Image-Diff-View-Modes/commit/8e95f70c9c47168305970e91021072673d7cdad8:

• images one 1 line
• smaller image centered both ways within the invisible bounding box the image would have had, if it was just as big
• size attributes be colored red and green respectively (think this is pretty nice)
• place the images always horizontal from each other, regardless of inline/side-by-side

cc: @ClemMakesApps