MDL-37245 Blog: Added permission check for blog comments

Conflicts:

	blog/index.php

Signed-off-by: Rajesh Taneja <rajesh@moodle.com>
MDL-37245 Blog: Fixed comments.

Conflicts:

	blog/lib.php

Signed-off-by: Rajesh Taneja <rajesh@moodle.com>

Conflicts:

	blog/lib.php