Improve editable content width adjustment behaviour in template editor

Currently, when trying to edit the width of editable content fields using the movable sliders, there becomes a point where the width is no longer adjustable when a placeholder is included. 

Steps to reproduce

• Create a custom template and in select a two column layout on a Documents section page

• In the right column add a placeholder

• Move the slider for the editable content field so that the width is reduced to the maximum

Actual result: Depending on the placeholder that is added, such as the Current Page Number placeholder, the max width will only allow the left column to take up between 90-92%.

Workaround: Remove the placeholder in the right column and move the slider for the desired width percentage for the left column (eg. 96-98%), then add the placeholder to the right column after.