Handle cases where subtitle fails to save to destination directory

This takes some of the logic of #3869 and adds some improvements. Basically it will always first download the remote subtitle to special://temp (or special://subtitles, if available) and try copy it from there to the movie's directory (if enabled). This is desirable as one can't always know upfront whether a certain directory will be writable.

Note that this also includes a fix for storing URL-encoded subtitle-files on the local filesystem.