Use capability filter for settings

[todeveni]

Description

Add a filter, so you can allow eg. editors to Able Player settings.

How Has This Been Tested?

By modifying current plugin source locally and with a filter in place. Settings show and are editable for editors.

add_filter(
  'ableplayer_capability',
  static function (): string {
    return 'edit_others_posts';
  }
);

Types of changes

Adds a capability filter

Checklist:

• My code is tested.
• My code is backward-compatible with WordPress 4.9 and PHP 7.0.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.

[joedolson]

I'm going to refactor this a bit so that it's assigned to a variable and add documentation for the filter, but this is a fine basis for that.