Footer layout refactoring and improvements for 0.9.0

[chalin]

This is a tracker issue so that we can make easier reference to all recent footer changes.

• layouts: split footer partial in several partials #1500
• Footer: factor out copyright, and code cleanup #1817
• The footer should allow a more flexible copyright statement #2
  • Footer, copyright notice: allow customization of year #1593
• Allow configuring the range of copyright years at the footer #436
  • Support date range in footer copyright element #1821
• Disable "footer_all_rights_reserved" by default #1840
• [Footer] Disable the "About..." link by default #1847
  • Footer streamlining: hide About-link and All-rights text by default #1849
• Copyright: default to "<Site.Title> Authors" #1850

Followup:

• Footer layout refactoring and improvements for 0.10.0 #1852

[chalin]

All done for 0.9.0. Other item move forward to:

• Footer layout refactoring and improvements for 0.10.0 #1852