Merge tag '2.1.7' into develop

Tagging version 2.1.7 2.1.7
[BUGFIX] Converter: Multilanguage forms can't be changed after conversion

remove two unset functions in FormConverter.php

related: #62146
[TASK] Added some more unit tests

SendMailTest.php
[TASK] Added another signal

SendMailTest.php
[FEATURE] Assign more stuff to $emailBodyObject for Mails

Two more variables in mail templates, a bit of code cleaning

related: #62653
[FEATURE] Fatal Error on visiting BE-Module

Remove usage of uriBuilder in LinkViewHelper to avoid problems with tq_seo

related: #62761
[FEATURE] Possibility to set default sender information

Add new TypoScript options, extended Div.php and FormController

related: #62728
[BUGFIX] Confirmation Message after send form doesn't appear

Redirect should only listen if hash GET param is not set

related: 62919
[BUGFIX] Confirmation Message after send form doesn't appear

Addes behavior test

related: 62919
Some code improvements
Manual update
Version update
Merge branch 'release/2.1.8'