Upstream changes
- Merge pull request 'Attempt calculating custom favicon avg color, only try calculating once' (#99) from wn/tt-rss:feature/custom-favicon-detect-color into master
- Minor tweak to favicon avg color debug log message.
- Don't set 'favicon_avg_color' on feed obj unless it's valid.
- Fix logging favicon-related bools in 'RSSUtils::update_rss_feed()'.
- Persist failure to detect favicon average color.
- Ensure custom favicon color detection happens.