[BUGFIX] Invalid form token when redirecting to EM

When Interactive Viewer is loaded without having EXT:restdoc,
a link to the Extension Manager is generated.

Since TYPO3 6.2 introduced an additional security layer,
following the link leads to an error message instead of
showing the EM.

Change-Id: I4aafd80276784b0dd6098d7e5feb7ae77f5b6340
Fixes: #56901
Reviewed-on: https://review.typo3.org/28377
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers