* Add compatibility for v3.1 checkout page

[MaximilianoRicoTabo]

All Submissions:

• [x] Have you followed the Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Add the gift aid checkbox on pmpro_checkout_boxes action. Tweak the pmproga_pmpro_checkout_after_level_cost function a bit.

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] 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.

[kimcoleman]

@MaximilianoRicoTabo please update this to use a structure more like https://github.com/strangerstudios/pmpro-payment-plans/blob/dev/pmpro-payment-plans.php#L436-L449. This should be a fieldset. We also need to us the <h2>tag, not <h3> here for accessibility / no skipped heading levels.

There is no "pmpro_normal" selector in our CSS.

The best way to see how a single checkbox filed should be shown is to add one in User Fields and reflect that same markup.

<div id="checkbox_field_div" class="pmpro_form_field pmpro_form_field-checkbox">
    <label class="pmpro_form_label pmpro_form_label-inline pmpro_clickable" for="checkbox_field">
        <input name="checkbox_field" type="checkbox" value="1" id="checkbox_field" checked="checked" class="pmpro_form_input pmpro_form_input-checkbox">
        Checkbox Field
    </label>
</div>

[MaximilianoRicoTabo]

done @kimcoleman