Import GitHub repositories respecting the API rate limit

While Octokit auto pagination set the page size to the maximum 100, and
seek to not overstep the rate limit. When the rate limit is reached its
raises an exception, and stop doing new requests.

Here we use a custom pattern for traversing large lists, so we can
check if we’ll reach the rate limit and wait the API to reset the rate
limit before making new requests.

lib/gitlab/github_import/pull_request_formatter.rb

@@ -24,6 +24,10 @@ module Gitlab
         }
       end
 
+      def create!
+        MergeRequest.create!(self.attributes)
+      end
+
       def number
         raw_data.number
       end