Fix over-sized editor pane widths
Fix over-sized editor pane widths

closes #5804, supersedes and closes #5820
- adds a fixed width that flexbox can expand from to prevent flexbox content dictating the width (see https://github.com/TryGhost/Ghost/issues/5804#issuecomment-141416812)
- adds a hack to the casperjs tests reverting the CSS change because phantomjs and flexbox don't get along

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