Add warning about deleting rule categories

Fixes #209

WIP - Need to update translations too.
Update Dutch and Spanish translations
Whitespace fix [ci skip]
Update French
Merge branch 'master' into issue/209

Conflicts:
	language/fr/boardrules_acp.php
Update Estonian
Merge pull request #216 from VSEphpbb/issue/209

Add warning about deleting rule categories