Implement Session support through the extension host (#3228)

Full plumb through of Session support. Also fixed some test issues

- Load session and get necessary information in kernels list
- Run Cell button now works as expected
- Added a ToggleAction base class which can be used for anything that switches icons. I'd still prefer to have this be dynamic and as clean as the extension classes
- Fixed account test unhandled promise rejections (caused by incorrect / invalid tests) that made it hard to see all the test run output.

src/sql/parts/notebook/outputs/common/outputProcessor.ts

@@ -40,7 +40,7 @@ export function getData(output: nb.ICellOutput): JSONObject {
             bundle['application/vnd.jupyter.stdout'] = output.text;
         }
     } else if (nbformat.isError(output)) {
-        let traceback = output.traceback.join('\n');
+        let traceback = output.traceback ? output.traceback.join('\n') : undefined;
         bundle['application/vnd.jupyter.stderr'] =
             traceback || `${output.ename}: ${output.evalue}`;
     }