[BUGFIX] Form email validation is case sensitive

Form email-validation is using a regular expression which is
case-sensitive.

Backporting the changes from 6.x would be a too large change.
But adding case-invalid check for the regex is okay.

Change-Id: Ib1449198f66f023a01c9791e74eb2d634abd77df
Resolves: #43924
Releases: 4.7
Reviewed-on: https://review.typo3.org/22117
Reviewed-by: Ernesto Baschny
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn