REUSE: Add Pleroma copyright

git diff --name-only 18593ba4109c6f3eecf63627ccae150bf911f091..pleroma/master | while read name; do test -e "$name" && reuse addheader --year 2019-2022 --copyright 'Pleroma Authors https://pleroma.social' --license 'LGPL-2.1-only' "$name"; done