Pull request preview URLs (#625)

* Dynamic link to HEAD of a given PR

* Display pr name and link in notification console

* Pass along query string to Explorer

This means you can write a URL like:

https://cosmos-explorer-preview.azurewebsites.net/pull/123?feature.enableFoo=true

and when Explorer loads it'll have enableFoo set to true in the features
object.

src/Platform/Hosted/extractFeatures.ts

@@ -19,6 +19,7 @@ export type Features = {
   readonly notebookServerUrl?: string;
   readonly sandboxNotebookOutputs: boolean;
   readonly selfServeType?: string;
+  readonly pr?: string;
   readonly showMinRUSurvey: boolean;
   readonly ttl90Days: boolean;
 };
@@ -57,6 +58,7 @@ export function extractFeatures(given = new URLSearchParams()): Features {
     notebookServerUrl: get("notebookserverurl"),
     sandboxNotebookOutputs: "true" === get("sandboxnotebookoutputs"),
     selfServeType: get("selfservetype"),
+    pr: get("pr"),
     showMinRUSurvey: "true" === get("showminrusurvey"),
     ttl90Days: "true" === get("ttl90days"),
   };