Install and configure Nextcloud server as www-data

Apache sub-processes are run as the www-data user, and they need to be
able to write to the "apps", "config" and "data" directories, so they
have to belong to that user, and therefore the Nextcloud server has to
be installed and configured too as the www-data user. The PHP built-in
web server will still be run as the root user, but in that case the
owner of those directories makes no difference, so this is compatible
with both cases.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>