Conditionals and Notebook UI fixes (#2868)

- Significant fixes for Safari-compatible Flex layout in Condition Set
view;
- Changed visual approach to current-value section;
- Firefox scrollbar coloring
- Fix layout issues in Firefox;
- Consolidate Conditionals styles into single scss file;
- Fix test datum elements layout, better wrapping;
- Better approach to presence/absence of URL property in Condition
Widget;
- Fixes #2853;
- Fix errors in URL property handling in Condition Widget;
- Fixes #2853;
- Fixes #2867 - hide the View Switcher when an object is being edited;
- Refined titling on View Switcher and Notebook menu button;
- Cleaned up styles in l-browse-bar and moved into
ui/layout/layout.scss;
- Removed styles/_layout.scss;
- Hide the main view Edit button when in mobile

src/plugins/themes/espresso-theme.scss

@@ -13,7 +13,6 @@
 @import "~styles/controls";
 @import "~styles/forms";
 @import "~styles/table";
-@import "~styles/layout";
 @import "~styles/legacy";
 @import "~styles/legacy-plots";
 @import "~styles/plotly";

src/plugins/themes/maelstrom-theme.scss

@@ -13,7 +13,6 @@
 @import "~styles/controls";
 @import "~styles/forms";
 @import "~styles/table";
-@import "~styles/layout";
 @import "~styles/legacy";
 @import "~styles/legacy-plots";
 @import "~styles/plotly";

src/plugins/themes/snow-theme.scss

@@ -13,7 +13,6 @@
 @import "~styles/controls";
 @import "~styles/forms";
 @import "~styles/table";
-@import "~styles/layout";
 @import "~styles/legacy";
 @import "~styles/legacy-plots";
 @import "~styles/plotly";