chore: Improve Helpcenter custom domains (#5456)

- Support rendering articles over frontend URL
- Support rendering articles over help center URL
- Support rendering help center home page in the custom domain root

db/migrate/20220919225556_make_help_center_custom_domain_unique.rb

@@ -0,0 +1,5 @@
+class MakeHelpCenterCustomDomainUnique < ActiveRecord::Migration[6.1]
+  def change
+    add_index :portals, :custom_domain, unique: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2022_08_09_104508) do
+ActiveRecord::Schema.define(version: 2022_09_19_225556) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
@@ -708,6 +708,7 @@ ActiveRecord::Schema.define(version: 2022_08_09_104508) do
     t.datetime "updated_at", precision: 6, null: false
     t.jsonb "config", default: {"allowed_locales"=>["en"]}
     t.boolean "archived", default: false
+    t.index ["custom_domain"], name: "index_portals_on_custom_domain", unique: true
     t.index ["slug"], name: "index_portals_on_slug", unique: true
   end