Separate high contrast theme (#7213)

app/javascript/styles/contrast.scss

app/javascript/styles/contrast/diff.scss

app/javascript/styles/contrast/variables.scss

app/javascript/styles/mastodon/about.scss

app/javascript/styles/mastodon/accounts.scss

app/javascript/styles/mastodon/admin.scss

app/javascript/styles/mastodon/compact_header.scss

app/javascript/styles/mastodon/components.scss

app/javascript/styles/mastodon/containers.scss

app/javascript/styles/mastodon/emoji_picker.scss

app/javascript/styles/mastodon/forms.scss

app/javascript/styles/mastodon/landing_strip.scss

app/javascript/styles/mastodon/stream_entries.scss

app/javascript/styles/mastodon/variables.scss

config/locales/en.yml

config/themes.yml