Raspiblitz custom installs

* default:
```
cat /mnt/hdd/app-data/custom-installs.sh
```
```
#!/bin/bash

# This script runs with sudo rights after an update/recovery from a fresh sd card.
# This is the place to put all the install commands, cronjobs or editing of system configs
# for your personal modifications of RaspiBlitz

# note: use absolute paths if you point to specific files

echo "There are no custom user installs so far."
```