convert: The never_convert_lossy_files config option should flip to convert_lossy_files

[RollingStar]

Problem

Convert: never_convert_lossy_files is one of the only config options I see in beets that uses negative wording. Instead of never_convert_lossy_files: no, the default could be convert_lossy_files: yes

[sampsyo]

Indeed; I've never really liked the name of this config option. I even think convert_lossy would be a fine name for the "reversed" version of this option.

If we change this, it will be important to preserve backwards compatibility by checking for an (undocumented) option with the original name and inverting it.