MDL-37977 Moved HTML from index.php to course renderer

- created functions core_course_renderer::frontpage_summary() and core_course_renderer::frontpage()
MDL-37977 minor improvements to frontpage renderer

- separated content of the frontpage news in its own function frontpage_news();
- coding style improvement
- editsection does not offer to choose a name for frontpage because it is not displayed anyway
MDL-39119 some functions moved from format_section_renderer_base to core_course_renderer
MDL-39119 function format_section_renderer_base::section_edit_controls() moved to course format
MDL-39119 upgrade.txt
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