Log Message:
Update to 3.8.6

Changelog:
Bug Fixes
    Update log output for an invalid theme directory (#7734)
    Memoize SiteDrop#documents to reduce allocations (#7722)
    Excerpt handling of custom and intermediate tags (#7467)
    Escape valid special chars in a site's path name (#7573)
    Revert memoizing Site#docs_to_write and refactor #documents (#7689)
    Fix broken include_relative usage in excerpt (#7690)
    Install platform-specific gems as required (3c06609)

Security Fixes
    Theme gems: ensure directories aren't symlinks (#7424)