[SECURITY] Prevent editor controlled hmac content

An hmac of the editor controlled auto respond message was used to verifiy
the correctness of this message on submit. To prevent this, we add an
additional secret.

Change-Id: I1551feebd4dd84abeb3fb098175384f425f605a9
Fixes: #45043
Releases: 4.5, 4.7, 6.0, 6.1, 6.2
Security-Commit: 344975268f4b9eb4ce7c664958647b9268ea03a8
Security-Bulletin: TYPO3-CORE-SA-2013-004
Reviewed-on: https://review.typo3.org/26223
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader