Werk #8392: Allow to add images to the normal element flow of a report

Up to now images could only be added as fixed page elements (useful for logos, etc.). Now you can put an image into the normal flow of elements. This allows you e.g. to embedd data from a third party application by creating some automated export into an image and placing that image under /omd/sites/mysite/local/share/check_mk/reporting/images/.

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