Allow to install with a custom user

[jooola]

While a variable to change the libretime user installed on the system is available, using a custom user is not yet working.

When using a custom user, some files should be readable by the web user (www-data), some dir should be writable.

Using php-fpm could make it easier for handling upload for example, as apache and mod_php does not let us use a custom user for the php process.

Needs some research.

[jooola]

Upstream ticket libretime/libretime#1637