Replace custom author byline with ABlog postcard sidebar

[niksirbi]

Reverts the custom author byline introduced in #496 and replaces it with ABlog's built-in postcard sidebar templates, following the same approach used in neuroinformatics-unit/movement#990. See discussion under the movement PR for context on how we reached this decision.

Preview.

Changes

• Remove the add_blog_author_byline Sphinx extension hook and its imports from conf.py
• Remove the .blog-post-author CSS block from custom.css
• Add html_sidebars config to use ABlog's native postcard.html + recentposts.html templates on blog post pages, and authors.html + archives.html on the blog index
• Set fontawesome_included = True so ABlog renders icons via the FontAwesome already bundled by the PyData theme