GitHub activity: allow sorting by an arbitrary attribute

This patch extends the GitHub activity sidebar widget such that one can sort by an arbitrary repo property (default `pushed_at`), say `stargazers_count` and in both ascending and descending order.

This one replaces the pull request https://github.com/getpelican/pelican-themes/pull/173. Being quite proficient at other VCS, I still haven't understood how to amend an existing pull request s.t. it arrives as a single commit.