[Meta] Untangle Drupal 8 page rendering

Problem/Motivation

Proposed resolution

Remaining tasks

User interface changes

API changes

During the DrupalCon Amsterdam discussion about page rendering, we decided that the following steps are necessary to fix #2327277: [Meta] Page rendering meta discussion:

Critical

1. Remove HtmlFragment& HtmlPage, but make sure that '#type' => 'page' works (because controllers should be able to return entire pages, without having the content be surrounded by blocks) — see #2352155: Remove HtmlFragment/HtmlPage
2. Prevent crazy asset handling and crazy page manipulation:
   1. For the valid use cases of adding content to the page, we introduce hook_page_top() and hook_page_bottom(): for adding content to the very top or bottom of the page (not regions!) — see #2350949: Add hook_page_attachments(_alter)() and deprecate hook_page_build/alter()
   2. Introduce hook_page_attachments(), which may be implemented by both modules and themes, to allow assets to be added to pages conditionally. For assets unconditionally added to pages by themes, hook_preprocess_page() must be used. We keep hook_page_build() around, for backwards compatibility — but we only allow assets to be added — see #2350949: Add hook_page_attachments(_alter)() and deprecate hook_page_build/alter()
3. Document how to attach assets correctly:
   1. Allow preprocess functions to cleanly attach assets for the associated template — see #2346369: Support special '#attached' variable for attaching assets in preprocess functions
   2. Documentation: hook_page_attachments() to conditionally attach assets to pages, hook_preprocess_page() to unconditionally attach assets to pages — see #2350949: Add hook_page_attachments(_alter)() and deprecate hook_page_build/alter()
4. Remove drupal_add_html_head(), drupal_add_html_head_link() and drupal_add_feed(), accept ['#attached']['head'], ['#attached']['head_link'], ['#attached']['feed'] instead — see #2350877: Deprecate/rename drupal_add_feed(), drupal_add_html_head(), drupal_add_html_head_link(), drupal_add_http_header(), and allow to be set declaratively in #attached

Nice-to-have

• Convert all page template (page.html.twig) variables to blocks
  • #2005546: Modify regions to support Convert site elements (site name, slogan, site logo) into blocks
  • #507488: Convert page elements (title, tabs, actions) into blocks
  • #2289917: Convert "messages" page element into blocks
• Extract RenderableInterface extends CacheableInterface from BlockPluginInterface. Begin with just a build() method that returns a render array, during D8 cycle, gradually add more methods so that the render array doesn't contain asset metadata anymore, etc. — until nothing remains, at which point we'll have done a lot of the work of moving away from the Render ArrayPI.