fix(background-sync): clean up open PRs that are deleted on GitHub

Pull requests cannot be deleted via API, only internally by GitHub staff.
This may lead to PR references to be "dangling". The resolution is to
treat all open PRs that could not be fetched as "upstream deleted".

Worst case these PRs will re-appear onto the board on PR activity, or
next background sync.

Closes #216

packages/app/lib/apps/background-sync/BackgroundSync.js

@@ -171,7 +171,7 @@ We automatically synchronize all repositories you granted us access to via the G
               response => response.data.filter(issue => !('pull_request' in issue))
             ),
 
-            // open pulls
+            // open pulls, all
             octokit.paginate(
               octokit.pulls.list,
               {
@@ -324,12 +324,23 @@ We automatically synchronize all repositories you granted us access to via the G
       let expired = false;
       let removed = false;
 
+      // if an open issue or pull request links to
+      // a non-existing repository, then it must have been
+      // removed
       if (!repositories[issue.repository.id]) {
         log.debug({ issue: key }, 'cleanup -> repository removed');
 
         removed = true;
       }
 
+      // if an open pull request was not found (we always
+      // fetch all), then it must have been deleted
+      if (issue.pull_request && issue.state === 'open') {
+        log.debug({ issue: key }, 'cleanup -> pull request deleted');
+
+        removed = true;
+      }
+
       const updatedTime = new Date(updated_at).getTime();
 
       if (updatedTime < expiryTime) {