[BUGFIX] Fix BS change of translationFile inclusions in TYPO3 10 but keep the old setting for TYPO3 9 compatibility

The userFunc was removed with dc00dbc1.
It used internal TYPO3 functions to display a simple list of the
available mail templates, so deleting the setting entirely should have no negative impact.

Feature upgrade to typo3 10

See merge request !30

[TASK] Remove exclude for l10n_parent field, because it causes bugs with translations in limited be-accounts

This reverts commit 5b9ee9f1, reversing
changes made to bb6c3845.

[BUGFIX] replace newlines in mail layout too

See merge request !28

Newlines were only replaced with <br>'s in the mail body, but not in the configured layout for this mail. This resulted in missing newlines in the layout (everything was more or less in one line).