Generates discussion_id when importing from GitHub

The GitHub importer skips callbacks so the discussion_id isn't
generated. This is fine because we also generate it at runtime when the
note is rendered.

But it is still better to generate it and persist it so that a note does
not have a different discussion id every time it is rendered.

The discussion_id is also going to be used for paginating discussions.

lib/gitlab/github_import/importer/note_importer.rb

@@ -29,6 +29,7 @@ module Gitlab
             project_id: project.id,
             author_id: author_id,
             note: note_body,
+            discussion_id: note.discussion_id,
             system: false,
             created_at: note.created_at,
             updated_at: note.updated_at

lib/gitlab/github_import/representation/note.rb

@@ -63,6 +63,14 @@ module Gitlab
           @attributes = attributes
         end
 
+        def discussion_id
+          Discussion.discussion_id(
+            Struct
+            .new(:noteable_id, :noteable_type)
+            .new(noteable_id, noteable_type)
+          )
+        end
+
         alias_method :issuable_type, :noteable_type
 
         def github_identifiers

spec/lib/gitlab/github_import/importer/note_importer_spec.rb

@@ -52,6 +52,7 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
                   project_id: project.id,
                   author_id: user.id,
                   note: 'This is my note',
+                  discussion_id: match(/\A[0-9a-f]{40}\z/),
                   system: false,
                   created_at: created_at,
                   updated_at: updated_at
@@ -82,6 +83,7 @@ RSpec.describe Gitlab::GithubImport::Importer::NoteImporter do
                   project_id: project.id,
                   author_id: project.creator_id,
                   note: "*Created by: alice*\n\nThis is my note",
+                  discussion_id: match(/\A[0-9a-f]{40}\z/),
                   system: false,
                   created_at: created_at,
                   updated_at: updated_at