MDL-40699 Language - Add option to Override Moodle's default behaviour to reset the UI language on logout

This change adds an option to the Site Admin > Langauge > Language Settings page which can prevent Moodle from changing language when users log out.

Affected files:
	lang/en/admin.php
	admin/settings/language.php
	login/logout.php

Signed-off-by: Michael Milette <michael.milette@instruxmedia.com>