Fix migration failure when groups are missing route

When a Group is missing a route, the migration failed previously with a
`Invalid single-table inheritance type` error. To fix this, we can
disable STI for the migration class because we don't need to know about
the type to do this migration. Besides, currently Group is the only type
used in the type column.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58714

changelogs/unreleased/sh-fix-issue-58714.yml

+---
+title: Fix migration failure when groups are missing route
+merge_request: 29022
+author:
+type: fixed

db/migrate/20180702134423_generate_missing_routes.rb

@@ -98,6 +98,7 @@ class GenerateMissingRoutes < ActiveRecord::Migration[4.2]
 
   class Namespace < ActiveRecord::Base
     self.table_name = 'namespaces'
+    self.inheritance_column = :_type_disabled
 
     include EachBatch
     include GenerateMissingRoutes::Routable

spec/migrations/generate_missing_routes_spec.rb

@@ -8,7 +8,7 @@ describe GenerateMissingRoutes, :migration do
     let(:routes) { table(:routes) }
 
     it 'creates routes for projects without a route' do
-      namespace = namespaces.create!(name: 'GitLab', path: 'gitlab')
+      namespace = namespaces.create!(name: 'GitLab', path: 'gitlab', type: 'Group')
 
       routes.create!(
         path: 'gitlab',