Center simple image slider items in Prettyblock

[CySSoO]

Motivation

• Fix centering of images in the Prettyblock "Simple Image" slider when the slider mode is active so images appear visually centered.

Description

• Add a dedicated wrapper class ever-slider--simple-image to the slider in views/templates/hook/prettyblocks/prettyblock_img.tpl and scope CSS rules in views/css/everblock.css to center .ever-slider-item media by applying text-align: center, display: block and margin-left/right: auto to picture and img.

Testing

• No automated tests were executed for this change.

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Codex Task