docs: replace issue hashes with links in changelog
docs: replace issue hashes with links in changelog

Github automatically adds links on release pages, but regular markdown
rendering when browsing the changelog file requires markdown links.

chhe/livestreamer-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui,streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui