[BUGFIX] Improve "Sign in with Twitter" process

The patch adds support for TYPO3 installations in subfolders and
corrects the submitted return url. Additionally the javascript was
improved to fill in retrieved token information automatically.

Change-Id: I79749209a081b4066c2db568edff12320a607b4a
Reviewed-on: https://review.typo3.org/26734
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes