Change the TYPO3 AdditionalConfiguration.php template to extend the MAIL array - fixes #1042

[dmnkhhn]

The Problem/Issue/Bug:

Problem described in #1042

How this PR Solves The Problem:

The code in the template file is changed so that the array is extended instead of overwritten so that we have access to the full $GLOBALS['TYPO3_CONF_VARS']['MAIL'] array in TYPO3.

Manual Testing Instructions:

Testing this automatically was not possible as far as I know.

[CLAassistant]

All committers have signed the CLA.

[rfay]

This seems totally reasonable if it tests out. I am in the dark about what gets lost this way, but still this is fine (after manual test).

Note that still the contents of AdditionalConfiguration.php can be changed on any project, just remove the #ddev-generated comment and ddev won't touch it any more.

[dmnkhhn]

There should be no negative impact as all other smtp settings are empty by default and they are not touched by this change.
There is only one MAIL setting that's not empty which is �transport_sendmail_command� and it is ignored because the transport is already set to smtp.

[rfay]

@dmnkhhn please do add the email you used for this commit to github per #1043 (comment) (And accept the CLA)- Thanks!

[dmnkhhn]

Done, thanks for the reminder!