Merge tag '2.1.8' into develop

Tagging version 2.1.8 2.1.8
[BUGFIX] Do not use "use" stmt in ext_localconf

removed use statement in ext_localconf.php and use full namespace-names for class call of ExtensionUtility

related: #63118
[FEATURE] Add new Database Check in Backend-Module

Added two new actions to moduleController. Some database function can check fields and forms on problems.

related: #63302
[FEATURE] Show Version (and security note in function overview in backend module)

Added some new viewHelpers and a new view to CheckBe.html

related: #63302
[FEATURE] Development Context to send only to one Email for testing

related: #63297
[FEATURE] Add TCA layout field to captcha field

related: #63317
[BUGFIX] Form Converter uses variables like {UID123} - but {uid123} would be correct

related: #63020
[BUGFIX] Call on a non object

related: #63028
[BUGFIX] uploaded picture gets deleted/removed when editing mail in frontend

related: #63152
[BUGFIX] Marker of localized fields should be empty

related: #63329
[BUGFIX] "own variable names" willl be destroyed using the "block"-modification via Web-Menu "LIST"

Fixed InitialMarker.php

related: #63046
[BUGFIX] Confirmation with Multilanguage doesn't work

set languageMode to NULL in findByUid() in MailRepository. This is only a small workarround. Child objects are still empty.

related: #62920
[BUGFIX] Unittest for getLocation changed

Google will now return street names in native language even if we force &language=en
[BUGFIX] Freegeoip fallback added

Un it tests shows that freegeoip.net was down today. Added telize.com as fallback solution
[TASK] Added Unit test for powermail version note

see description + refactored PowermailVersionNoteViewHelper.php
[TASK] Restructuring Documents on Documentation/ForAdministrators/GoodToKnow/*
[TASK] Added some more information to Documentation/Backend Module
[TASK] Documentation for development context added
[TASK] Documentation for development context added
[BUGFIX] Updated IP-Address for DivTest.php