Use plural for permanent delete posts/topics confirmation

When deleting (multiple) topics or posts from MCP, the confirmation label of the permanent delete does not use plural.
In order to fix this, the language variables:
L_DELETE_TOPIC_PERMANENTLY
L_DELETE_POST_PERMANENTLY
in phpBB/styles/prosilver/template/confirm_delete_body.html must be replaced with template variables (that handle plurality) in functions mcp_delete_topic and mcp_delete_post in phpBB/includes/mcp/mcp_main.php