UX: improve styling on full page search page
FEATURE: allow search context in full page search
FEATURE: visited color link for full page search
FIX: broken search help on fulls page search page
FEATURE: allow preload store to return a null
FEATURE: "mobileAction" for the header buttons
- Moves the import of plugins for both mobile and desktop from common after discourse loading, allowing plugins to overwrite
- Make desktop-option behave like the mobile-option: SCSS/CSS marked with that option will only be loaded for desktop from now on and ignored in mobile
- Add variables-keyword, allowing plugins to ship and overwrite variables before they get imported by discourse (great for theming)