Implement EZP-21191: Implement new internal format to cover ezxml capabilities

DO NOT MERGE

This PR relates to https://jira.ez.no/browse/EZP-21191

For the demo purposes a bundle is implemented, providing HTML5 editing with TinyMCE editor. Steps to enable `EzPublishXhtml5EditTestBundle` bundle are:

1. register the bundle with kernel
2. import routes
3. disable CSRF protection
4. disable basic auth for REST, so that session based auth is used instead
5. create content with XmlText field definition "test"
6. authenticate into frontend
7. open created content with `http://ezpublish.local/test/<contentId>`

Usable example on how to configure `youtube` custom tag (using test fixtures):

`ezpublish.yml`
```yaml
ezpublish:
    system:
        ezdemo_site_group:
            fieldtypes:
                ezxml:
                    output_custom_tags:
                        -
                            path: %kernel.root_dir%/../vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/FieldType/Tests/XmlText/Converter/Xslt/_fixtures/docbook/custom_stylesheets/docbook_to_xhtml5_output.xsl
                            priority: 1
                    edit_custom_tags:
                        -
                            path: %kernel.root_dir%/../vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/FieldType/Tests/XmlText/Converter/Xslt/_fixtures/docbook/custom_stylesheets/docbook_to_xhtml5_edit.xsl
                            priority: 1
                    input_custom_tags:
                        -
                            path: %kernel.root_dir%/../vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/FieldType/Tests/XmlText/Converter/Xslt/_fixtures/xhtml5/custom_stylesheets/xhtml5_edit_to_docbook.xsl
                            priority: 0
```