FEATURE: ability to skip site package installation. NEOS_APP_SITE_PACKAGE=false by default.

With this change NEOS_APP_SITE_PACKAGE is not set by default to 'TYPO3.NeosDemoTypo3Org' anymore. By default it is FALSE which means site package will not be installed. In the descendant images you have to specify NEOS_APP_SITE_PACKAGE if you want to install one.
Flow support pre-requisite: files reorganised, 'neos' part removed from all filenames
CircleCI: cosmetic improvements