Remove early rendering of RSS channel elements in template_preprocess_views_view_rss()

Problem/Motivation

template_preprocess_views_view_rss() has the following line:

  $variables['channel_elements'] = \Drupal::service('renderer')->render($style->channel_elements);

The channel elements are then printed in views-view-rss.html.twig:

<rss version="2.0" xml:base="{{ link }}"{{ namespaces }}>
  <channel>
    <title>{{ title }}</title>
    <link>{{ link }}</link>
    <description>{{ description }}</description>
    <language>{{ langcode }}</language>
    {{ channel_elements }}
    {{ items }}
  </channel>
</rss>

By forcing the {{ channel_elements }} to render early, its contents can no longer be changed by other modules. The Views RSS module for example extends the channel elements with more elements from the RSS spec: http://www.rssboard.org/rss-profile

This module now has to re-render the channel elements with the updated channel elements.

Proposed resolution

Remove early rendering of RSS channel elements in template_preprocess_views_view_rss() so other modules can add channel elements without having to re-render.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

{{ channel_elements }} in views-view-rss.html.twig now is a renderable array instead of rendered markup.

Data model changes

None.