Allow custom config file in LimeSurvey

[milian.hackradt]

For a lot of configurations in LimeSurvey, their own documentation suggests editing the config.php file directly. Right now, this is copied to /run/limesurvey from /app/pkg on each restart - it would be great if we could place a config.php in /app/data/ so changes persist across restarts.

The quickest way would probably be an edit to start.sh:

if [[ -f /app/data/config.php ]]; then
    cp /app/data/config.php /run/limesurvey/config.php
else
    cp /app/pkg/config.php /run/limesurvey/config.php
fi

This could potentially break on app updates, so it might make sense to document it as an "at your own risk" feature.