Resolves ENG-3679.
Used fetch to preload encrypted media. The current preloadImage function used to preload them by creating a dummy <img /> element and setting the src property, which was not suitable for encrypted media.
The introduced function in fact preloads any request, including images, but I left the original because browser might have some optimizations for images. Also it is used in other places in the codebase e.g. to get the dimensions.