[BUGFIX] Allow opening custom doktype in the frontend

A few doktype are checked now to open the frontend page.
Others will redirect to the homepage. Allow custom doktypes now
too with opening the frontend page.

Resolves: #75499
Resolves: #70094
Releases: master,7.6
Change-Id: Iaffe03dfff3f1181f26964f26611d80e71406b5c
Reviewed-on: https://review.typo3.org/48036


Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Zbigniew Jacko <z.jacko@macopedia.pl>
Tested-by: Zbigniew Jacko <z.jacko@macopedia.pl>
Tested-by: Marcin Krzyżanowski
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>