Most of the art/textures/ui/session/icons/mappreviews/ have snake_case filenames, however, a few have capitals, spaces, or parentheses, which is inconsistent and potentially problematic. This patch renames those png map previews and updates their corresponding maps/ files accordingly.
Furthermore, the names of the sandbox map previews were not always obvious (e.g. `sandbox-themacks.png`), therefore this proposal also applies the `sandbox_kushites.png` naming scheme to the others for consistency.