Generate legacy image placeholder text by taking first letters

https://github.com/nextcloud/server/issues/19335


Signed-off-by: Sergey Shliakhov <husband.sergey@gmail.com>
Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>
Signed-off-by: npmbuildbot[bot] <npmbuildbot[bot]@users.noreply.github.com>