Issue 1197 - consistent file edit permission check

Make the check for whether the 'edit files' admin page should be shown consistent - check was being done differently in three different places, now changed to consistent function call using `current_user_can()` as discussed in issue #1197 and adding a proprietary filter `wpseo_allow_system_file_edit` to overrule the outcome.

Also: remove `function_exists( 'is_multisite' )` checks everywhere as min requirement for the plugin is nowadays well above WP 3.0, the check for `function_exists` on `is_multisite` is no longer needed.