Fix empty settings page for users with `manage_options` capability without administrator role

[rvdsteege]

The WordPress admin settings page of the Cloudflare plugin is empty for users having the manage_options capability without the administrator role (as is the case with for example a custom 'Manager' role).

This PR intends to fix #544 which describes this issue, by using the same manage_options capability check for the admin menu page and the 'Cloudflare proxy':

The "Cloudflare" admin menu item requires the manage_options capability and the WordPress AJAX action cloudflare_proxy — which seems needed to load the settings page — is checking for the administrator role.

Can you take a look at this, @aseure?