Use only hash-based notations for data attributes
Remove unused and outdated as_button
Merge branch 'tidy-up-haml-notations'
Minify resulting HTML of /
Remove extra spaces within #header buttons
Remove extra whitespace within links
Separate #offline_mode .progress from icon
Merge branch 'minify-html'

Google PageSpeed Insights still suggests minifying HTML, but it's not
worth to reduce HTML size anymore, because reducible content is only
523 bytes.