Backfill route namespace_id for namespaces

What does this MR do and why?

Backfills route's namespace_id for user namespaces and groups (for these records it's just copy of source_id). Projects will be backfilled later (when project namespaces are backfilled for projects).

Related to #346446 (closed)

DB migration

== 20220120123800 BackfillNamespaceIdForNamespaceRoutes: migrating ============
== 20220120123800 BackfillNamespaceIdForNamespaceRoutes: migrated (0.0524s) ===

== 20220209073600 AddTmpIndexRoutesIdForNamespaces: migrating =================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:routes, :id, {:where=>"routes.namespace_id is null and routes.source_type = 'Namespace'", :name=>"tmp_index_for_namespace_id_migration_on_routes", :algorithm=>:concurrently})
   -> 0.0033s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:routes, :id, {:where=>"routes.namespace_id is null and routes.source_type = 'Namespace'", :name=>"tmp_index_for_namespace_id_migration_on_routes", :algorithm=>:concurrently})
   -> 0.0083s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220209073600 AddTmpIndexRoutesIdForNamespaces: migrated (0.0152s) ========

$ rake db:rollback
== 20220209073600 AddTmpIndexRoutesIdForNamespaces: reverting =================
-- transaction_open?()
   -> 0.0000s
-- indexes(:routes)
   -> 0.0030s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- remove_index(:routes, {:algorithm=>:concurrently, :name=>"tmp_index_for_namespace_id_migration_on_routes"})
   -> 0.0175s
-- execute("RESET statement_timeout")
   -> 0.0004s
== 20220209073600 AddTmpIndexRoutesIdForNamespaces: reverted (0.0242s) ========

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