Allow level selection on the [pmpro_signup] shortcode.

[louiswol94]

Two shortcode attributes added:
'levels' (string of level ids to show in selection, or "all" to show all levels, default NULL)
'levels_select' (string - default "dropdown", otherwise radio buttons will show)

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Changes proposed in this Pull Request:

This pull request adds the ability for customers to enable "level selection" on the [pmpro_signup] shortcode. This will enhance the user experience and increase conversion rates. I've had a request to do this for a customer and I thought it would be nice to have this option available in the plugin itself.

I added two attributes for the shortcode:

• 'levels': The editor to specify which levels should be included in the selection - either by giving "all" or level IDs "1,2,4,5,9". The default is NULL in which case nothing will change.
• 'levels_select': The editor to specify a dropdown select or radio button select.

The default required 'level' attribute will now just be the default selected/checked item.

I also disabled data attributes like $checkout_boxes, $intro and $title as they relate to an individual Level. But I will still display $intro and $title if custom strings were passed and not just "true".

How to test the changes in this Pull Request:

1. Test if the normal shortcode is still in tact: [pmpro_signup level="1" login="1" redirect="referrer"]
2. Show all levels in a dropdown: [pmpro_signup level="1" login="1" redirect="referrer" short="true" levels="all"]
3. Show specific levels in radio buttons: [pmpro_signup level="1" login="1" redirect="referrer" short="true" levels="1,3" levels_select="radio"]

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.
Enhancement: Optionally allow for level selection on the [pmpro_signup] shortcode.

[ideadude]

This is really cool, but we'll need to update this to match the latest version of the code. There are some sanitization and escaping issues as well. We'll have a look soon.