TS-97: Page: custom attributes now applied correctly to BODY element

Before custom attributes (mainly from M12.Foundation:AbstractDevBase) were applied correctly only to Content node types. This fix makes them working correctly also on Page node type.

Relates to TS-55, TS-57, TS-62, TS-109, TS-111.
TS-111, TS-55: TYPO3.Neos:Page: custom js/css now appended independently from site package

This eliminates the need to include these styles/js in the site package - they are now included always, on the TYPO3.Neos:Page level.

Additionally, they are now included at the *end* of BODY, not in HEAD section. This resolves the problem when the styles were not applied without complete reload of the backend.