Version bump and changelog for 0.19.1

changes/changelog.d/867.enhancement

-Added a SUBSONIC_DEFAULT_TRANSCODING_FORMAT env var to support clients that don't provide the format parameter (#867)

changes/changelog.d/868.enhancement

-Added opus to the list of supported mimetypes and extensions (#868)

changes/changelog.d/869.bugfix

-Fixed broken user admin for users with non-digit or letters in their username (#869)