IMAP auth added:
- Standard.php extended, having imap_connection saved into baikal.yaml config file & Webadmin UI got a new conneted field to read/modify the IMAP connection
- Server.php modified, to be able to use imap_connection from baikal.yaml

Including Sabre.io/dav v4.6.0 IMAP Auth code, some tweaking

The web-admin UI now handles the additional IMAP connection input field as it is in the database settings page: when IMAP selected the field rendered, if not it gots hidden.

(No validation built-in)

Fix: remove declaration of non-existent validation-hook

The unchanged IMAP.php from sabre.io/dav got updated:
- the Baikal namespace & class extension
- the GitHub PR at Baikel using different PHP-CS requirements than the one at Sabre.io/dav -> CS has been aligned to Baikal's one

The same CS alignation applied to:
- Core/Server.php
- Model/Config/Standard.php
- Controller/Settings/Standard.php