* Add extension-landing generalized styles, install scratch link component
* Make EV3 page use generalized stuff
* Use OS_ENUM file instead of class variable
* use extension-landing/os-enum in os chooser
* Use extension-landing class in extension-landing.scss
* Use extension-landing styles and components on microbit page
* Add view-specific styles
* Move install scratch link l10n strings to src/l10n.json
* Start moving steps display to its own components
* Finish initial pass at Step, Steps components for extension landing pages
* Create ProjectCard component
* Use new components on InstallScratchLInk component
* Use new components on EV3 page
* allow className prop in Steps component
* Use new components on micro:bit landing page
* imageUrl -> imageSrc in ProjectCard
* Create ExtensionHeader component and use it on micro:bit and EV3 pages
* Fix a spacing issue in the InstallScratchLink component
* Add ExtensionRequirements component
* Use ExtensionRequirements component on landing pages
* Remove requirements l10n string for ev3 page
* Move project card styles out of things-to-try section
* Don't render the number row in a step if compact and number props are not set
* Add ExtensionSection component
* Use ExtensionSection on ev3 and microbit pages
* Move state configuration to ExtensionLanding class
* Move tip box, screenshot styles outside of specific section
* Add TipBox component and use it on the EV3 page
* Use hr element instead of section-separator div
* Remove refactor TODO comments :)
Updated all the places in the SCSS where we use Darken and Lighten properties.
These properties have been problematic in the past and caused some state issues, e.g. New message(s) hightlight (which was fixed seperately from this issue)
