Replace $GLOBALS with Context API and Aspects for TYPO3 v9

[benwalch]

IN TYPO3 v9, $GLOBALS['TSFE'] is deprecated.

For the appropriate functions in the Plugin, use the new Context API:
https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ApiOverview/Context/Index.html

For example $GLOBALS['TSFE']->fe_user is now $context->getAspect('frontend.user')