GT-2290 Support a pagecollection page type

[frett]

new XML:

page collection page

<page xmlns="https://mobile-content-api.cru.org/xmlns/page"
    xmlns:analytics="https://mobile-content-api.cru.org/xmlns/analytics"
    xmlns:content="https://mobile-content-api.cru.org/xmlns/content"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="page-collection" id="RG_page_collection">
    <analytics:events>
        <analytics:event action="test" system="firebase" />
    </analytics:events>
    <pages>
        <import filename="Gifts01.json" />
        <import filename="Gifts02.json" />
        <import filename="Gifts03.json" />
        <page xsi:type="content" id="embedded_page">
            <analytics:events>
                <analytics:event action="test" system="firebase" />
            </analytics:events>
            <content>
                <content:spacer />
                <content:paragraph>
                    <content:text>Text</content:text>
                </content:paragraph>
            </content>
        </page>
        <import filename="Gifts04.json" />
        <import filename="Gifts05.json" />
        <import filename="Gifts06.json" />
        <import filename="Gifts07.json" />
    </pages>
</page>

page parent updates

<!-- long term how parent pages can be specified to deep link to a specific page in a page collection -->
<page cyoa:parent="RG_page_collection?active-page=RG03" />

<!-- For backwards compatibility while we need to support older versions of the app that don't support page collections -->
<page cyoa:parent="RG03" cyoa:parent_override_page-collection="RG_page_collection?active-page=RG03" />