Fix kernel name check bug, double-event hooking, and other Notebook issues (#4016)

* Fix error where kernel name was compared to itself. This doesn't break anything right now since we happen to have special handling of Python3, and for other kernels they share the same set of supported providers (which is what the check is used for). However it needs fixing for next release.
* Fix console error due to queryTextEditor trying to access model before it's ready
* Fix issues where notebook model hooked to session events multiple times
* Removed calls that weren't needed such as loadActiveContexts (if undefined, does nothing) and passing connection to initialize method

src/sql/parts/modelComponents/queryTextEditor.ts

@@ -131,6 +131,10 @@ export class QueryTextEditor extends BaseTextEditor {
 			this._scrollbarHeight = this._config.editor.viewInfo.scrollbar.horizontalScrollbarSize;
 		}
 		let editorWidgetModel = editorWidget.getModel();
+		if (!editorWidgetModel) {
+			// Not ready yet
+			return;
+		}
 		let lineCount = editorWidgetModel.getLineCount();
 		// Need to also keep track of lines that wrap; if we just keep into account line count, then the editor's height would not be
 		// tall enough and we would need to show a scrollbar. Unfortunately, it looks like there isn't any metadata saved in a ICodeEditor