Improve notebook colors and UX (#3914)

This was reviewed / worked on with Smitha and will be signed off on by PM via mail.
1 thing left (make run button look better when not selected) will be one in separate review.

Changes
- Add top/bottom padding to editor so it's not cramped
- Added an (on by default) setting `notebook.overrideEditorTheming`. This controls whether new colors etc. are used for notebook editors or if users should see vanilla UI like in standard editor. Settings under this flag are:
  - When unselected, editor has same color as toolbar. On selection it goes back to regular editor view so colors work "right"
  - In standard light/dark themes we now use a filled in background color instead of border box.

src/sql/parts/notebook/cellViews/code.css

@@ -41,6 +41,9 @@ code-component .toolbarIconStop {
 	background-image: url('../media/dark/stop_cell_inverse.svg');
 }
 
+code-component .editor {
+	padding: 5px 0px 5px 0px
+}
 /* overview ruler */
 code-component .monaco-editor .decorationsOverviewRuler {
 	visibility: hidden;