Add widget area in header

[benlk]

Changes

• Adds filters in partials/largo-header.php and docs in the dev docs:
  • largo_header_before_largo_header: run immediately before largo_header
  • largo_header_after_largo_header: run immediately after largo_header
• Adds new widget area "Header Widget":
  • enabled by a theme option, defaults to off
  • is output by largo_header_widget_sidebar
  • Has default styles floating it right and limiting the width to 320px.

Why

For VO-17.

Takes care of part of #1038, making it easier to place widgets in this area.