Fix errors due to icon to codicon rename in VSCode (#7837)

Found numerous errors where icons weren't rendering correctly.
- Anything that's an action must have CSS using "codicon" not "icon"
since VSCode sets the "codicon" class automatically.
This affected Agent view in particular, but also:
  - acounts view
  - new tab view in dashboard
 - many more
- Anything referencing the common-icons.css icons needed updating.
This hid help tooltip text in FormContainer UI for example.
- Finally I tried to convert all references from icon -> codicon,
even when the CSS was technically correct. This was done
for maintainability reasons - from now on always add codicon.

Fixes #7827

src/sql/base/browser/ui/panel/tabHeader.component.ts

@@ -58,7 +58,7 @@ export class TabHeaderComponent extends Disposable implements AfterContentInit,
 
 		let tabLabelcontainer = this._tabLabelRef.nativeElement as HTMLElement;
 		if (this.showIcon && this.tab.iconClass) {
-			tabLabelcontainer.className = 'tabLabel icon';
+			tabLabelcontainer.className = 'tabLabel codicon';
 			tabLabelcontainer.classList.add(this.tab.iconClass);
 		} else {
 			tabLabelcontainer.className = 'tabLabel';