Refactor Project/Group import missing author note

- Removes duplicated `admin?` check. This validation is already done in
  Gitlab::ImportExport::MembersMapper.
- Remove some accessory methods, making the conditional more explicit.
- Use String template to manipulate the note itself

lib/gitlab/import_export/base/relation_factory.rb

@@ -248,25 +248,17 @@ module Gitlab
           old_author_id = @original_user['author_id']
           author = @relation_hash.delete('author')
 
-          update_note_for_missing_author(author['name']) unless has_author?(old_author_id)
-        end
-
-        def has_author?(old_author_id)
-          admin_user? && @members_mapper.include?(old_author_id)
+          unless @members_mapper.include?(old_author_id)
+            @relation_hash['note'] = "%{note}\n\n %{missing_author_note}" % {
+              note: @relation_hash['note'].presence || '*Blank note*',
+              missing_author_note: missing_author_note(@relation_hash['updated_at'], author['name'])
+            }
+          end
         end
 
         def missing_author_note(updated_at, author_name)
           timestamp = updated_at.split('.').first
-          "\n\n *By #{author_name} on #{timestamp} (imported from GitLab project)*"
-        end
-
-        def update_note_for_missing_author(author_name)
-          @relation_hash['note'] = '*Blank note*' if @relation_hash['note'].blank?
-          @relation_hash['note'] = "#{@relation_hash['note']}#{missing_author_note(@relation_hash['updated_at'], author_name)}"
-        end
-
-        def admin_user?
-          @user.admin?
+          "*By #{author_name} on #{timestamp} (imported from GitLab)*"
         end
 
         def existing_object?