Use GitHub Issue/PR number as iid to keep references

With these changes we don’t lost the issue/pr references when importing
them to GitLab.

app/models/concerns/internal_id.rb

@@ -7,12 +7,14 @@ module InternalId
   end
 
   def set_iid
+    if iid.blank?
       records = project.send(self.class.name.tableize)
       records = records.with_deleted if self.paranoid?
       max_iid = records.maximum(:iid)
 
       self.iid = max_iid.to_i + 1
     end
+  end
 
   def to_param
     iid.to_s

lib/gitlab/github_import/issue_formatter.rb

@@ -3,6 +3,7 @@ module Gitlab
     class IssueFormatter < BaseFormatter
       def attributes
         {
+          iid: number,
           project: project,
           title: raw_data.title,
           description: description,

lib/gitlab/github_import/pull_request_formatter.rb

@@ -3,6 +3,7 @@ module Gitlab
     class PullRequestFormatter < BaseFormatter
       def attributes
         {
+          iid: number,
           title: raw_data.title,
           description: description,
           source_project: source_project,

spec/lib/gitlab/github_import/issue_formatter_spec.rb

@@ -30,6 +30,7 @@ describe Gitlab::GithubImport::IssueFormatter, lib: true do
 
       it 'returns formatted attributes' do
         expected = {
+          iid: 1347,
           project: project,
           title: 'Found a bug',
           description: "*Created by: octocat*\n\nI'm having a problem with this.",
@@ -50,6 +51,7 @@ describe Gitlab::GithubImport::IssueFormatter, lib: true do
 
       it 'returns formatted attributes' do
         expected = {
+          iid: 1347,
           project: project,
           title: 'Found a bug',
           description: "*Created by: octocat*\n\nI'm having a problem with this.",

spec/lib/gitlab/github_import/pull_request_formatter_spec.rb

@@ -35,6 +35,7 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
 
       it 'returns formatted attributes' do
         expected = {
+          iid: 1347,
           title: 'New feature',
           description: "*Created by: octocat*\n\nPlease pull these awesome changes",
           source_project: project,
@@ -58,6 +59,7 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
 
       it 'returns formatted attributes' do
         expected = {
+          iid: 1347,
           title: 'New feature',
           description: "*Created by: octocat*\n\nPlease pull these awesome changes",
           source_project: project,
@@ -81,6 +83,7 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
 
       it 'returns formatted attributes' do
         expected = {
+          iid: 1347,
           title: 'New feature',
           description: "*Created by: octocat*\n\nPlease pull these awesome changes",
           source_project: project,