Add background image support to Everblock repeater

[CySSoO]

Motivation

• Allow editors to upload a background image per item in the PrettyBlocks repeater for Everblock so repeaters can use an image background in addition to color.

Description

• Add a background_image repeater field (type: 'fileupload') to the Everblock PrettyBlocks block configuration in src/Service/EverblockPrettyBlocks.php.
• Render the uploaded image in the repeater template by injecting background-image:url(...) with background-size: cover; background-position: center; background-repeat: no-repeat; in views/templates/hook/prettyblocks/prettyblock_everblock.tpl.

Testing

• No automated tests were executed for this change.

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Codex Task