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/notebook.component.ts

@@ -3,8 +3,6 @@
 *  Licensed under the Source EULA. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/
 
-import 'sql/parts/notebook/notebookStyles';
-
 import { OnInit, Component, Inject, forwardRef, ElementRef, ChangeDetectorRef, ViewChild, OnDestroy } from '@angular/core';
 
 import { IColorTheme, IWorkbenchThemeService } from 'vs/workbench/services/themes/common/workbenchThemeService';