Update label view helper to have html escape by default

This PR replicates the `FormRow` view helper logic, which escapes the `Element` `label` property by default and takes advantage of the new `disable_html_escape` `labelOptions` property added in #4677 to toggle the escape off when needed.

This may look like a minor BC break, because anyone who used `FormLabel` helper rather than `FormRow` will now have unexpected escaping, but since the overall policy of ZF2 is 'secure by default' I would rather consider this a minor security fix.