A first step to adding automatic dark mode color scheme switching. Adds a new SCSS file at `color_definitions.scss` that serves to output all SCSS color variables as CSS custom properties. And replaces all SCSS color variables with the new CSS custom properties throughout the stylesheets.
This is an alpha feature at this point, can only be enabled via console using the `default_dark_mode_color_scheme_id` site setting.
- moves footer nav to the header on iPads
- disables scrolling events for iPads
- removes footer nav entirely on Chrome PWAs
- toggles DiscourseHub iOS app status bar styling (dark/light) when opening/closing lightboxes and modals
