Internalize drupal_prepare_page()

Problem/Motivation

drupal_prepare_page() is called from only 3 places:

* DefaultHtmlPageRenderer, which is realistically the only place it should be called.
* drupal_render_page(), which is about to cease to exist: #2332551: Remove drupal_render_page()
* BatchController, which is really just duplicating the work of DefaultHtmlPageRenderer

Proposed resolution

* Move the logic of drupal_render_page() into a protected method of DefaultHtmlPageRenderer
* Modify BatchController to use DefaultHtmlPageRenderer directly
* Remove drupal_render_page()

This means we can refactor and even remove that logic within the scope it belongs to, that is, page rendering, aka DefaultHtmlPageRenderer.

Remaining tasks

Do it.

User interface changes

None.

API changes

A function you should never be touching directly at this point anyway goes away.