[FOLLOWUP][BUGFIX] Ensure absolute paths for &eID=dumpFile

This follow-up patch ensures the link is properly generated when
generating it from a Backend context, e.g., when embedding an image
within CKEditor and when TYPO3 is within a subdirectory.

Change-Id: I9783344c70c8cdfd6660a69dff72b251b2964c16
Resolves: #81486
Releases: master
Reviewed-on: https://review.typo3.org/53789


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>