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/sqlops.proposed.d.ts

@@ -1698,6 +1698,20 @@ declare module 'sqlops' {
 			 */
 			requestComplete(content: ICompleteRequest): Thenable<ICompleteReplyMsg>;
 
+			/**
+			 * Interrupt a kernel.
+			 *
+			 * #### Notes
+			 * Uses the [Jupyter Notebook API](http://petstore.swagger.io/?url=https://raw.githubusercontent.com/jupyter/notebook/master/notebook/services/api/api.yaml#!/kernels).
+			 *
+			 * The promise is fulfilled on a valid response and rejected otherwise.
+			 *
+			 * It is assumed that the API call does not mutate the kernel id or name.
+			 *
+			 * The promise will be rejected if the kernel status is `Dead` or if the
+			 * request fails or the response is invalid.
+			 */
+			interrupt(): Thenable<void>;
 		}
 
 		export interface IInfoReply {