Forum pages are adjusting breadcrumb too late

Follow-up to: #1861676: Remove stylesheets[] and scripts[] .info file property support for modules

Problem

• forum_page() cannot return a render array, because template_preprocess_forums() tries to override the breadcrumb, and the breadcrumb is rendered on the page before the preprocessor gets invoked.

Goal

• Move the breadcrumb override somewhere else, so that forum_page() can return a render array.