Add a message warning user that Webhooks will be disabled

app/views/import/github/status.html.haml

@@ -4,6 +4,10 @@
   %i.fa.fa-github
   Import projects from GitHub
 
+%p
+  %i.fa.fa-warning
+  To import GitHub pull requests, any pull request source branches that had been deleted are temporarily restored on GitHub. To prevent any connected CI services from being overloaded with dozens of irrelevant branches being created and deleted again, GitHub webhooks are temporarily disabled during the import process.
+
 %p.light
   Select projects you want to import.
 %hr