[BUGFIX] Backend typolink fields not saved

Adds a trailing slash on input and a call to onchange for that field.

Change-Id: Ib17216aba9c0190018a44a04e2081cac19c0b84a
Resolves: #25946
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/35622
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>