Popular v0.4.0

Added

• Astro package: popular({ routes: { speakers: false, ... } }) disables
  any injected route group, the supported way to replace part of the
  content model or provide your own / or /rss.xml.
• Astro package: injected slug routes use rest params, so folder-organized
  content ids (2019-pycon-us/cooper-lees) build.

Changed

• Astro: theme pages tolerate undefined or empty collections, rendering no
  pages instead of failing the build.
• Theme-only CI workflows (deploys, releases, parity checks and reminders)
  no longer run in forks of either repo.