Set z-index on status-bar when stuck by sticky directive

[rpeterman-gp]

• add strata variables to base SCSS
• update header, modal, and toastr to use SCSS
• update header, modal, and toastr styles to use strata and color variables
• add styling for button invalid tips in status bar

To review

• [x] Start up publish local docker dev on master branch
• [x] Run npm run build:lib:watch
• [x] Run CONTAINER_NAME=publishprxorg_publish npm run postbuild:lib:watch
• [x] Run npm install
• [x] Run npm run storybook
• [x] Go to Status Bar Usage Details
• [x] Inspect status bar and add class="js-stuck attribute to prx-status-bar element
• [x] Ensure the style adds a z-index to status bar
• [x] Go to publish.prx.docker
• [x] Ensure top nav, status bar, and forms stack as expected on any edit screen
• [x] Ensure modals and toasts stack as expected
• [x] Edit a series and clear one of the required fields.
• [x] Ensure save button shows an invalid tip below the status bar