Allow remaining images on DIMP main page to be loaded via Data URL

Allow remaining images on DIMP main page to be loaded via Data URL

Loading DIMP's index page on a supported browser, with CSS/JS caching,
now results in 0 images that need to be separately loaded.

Should probably move this to Horde core proper ASAP (along with all
JS/CSS caching code) so that other apps can make use of this. This is
less effective for non-AJAXy applications since the data needs to be
sent on every page load (vs. the image data being cached in CSS).
However, some images are generated on the fly/don't reside in the app
themes directory so they aren't available to be served via CSS.