Add namespace_id reference to routes

What does this MR do and why?

This reference will serve as a replacement of the existing source polymorphic reference (eventually we want to use only namespace routes).

Related to #349598 (closed)

Related discussion also in &7184 (comment 776564877)

DB Migration

$ rake db:migrate
== 20220105121325 AddRouteNamespaceReference: migrating =======================
-- column_exists?(:routes, :namespace_id)
   -> 0.0018s
-- add_column(:routes, :namespace_id, :bigint)
   -> 0.0008s
== 20220105121325 AddRouteNamespaceReference: migrated (0.0026s) ==============

== 20220107091629 AddRouteNamespaceIndex: migrating ===========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:routes, :namespace_id, {:unique=>true, :name=>"index_routes_on_namespace_id", :algorithm=>:concurrently})
   -> 0.0026s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:routes, :namespace_id, {:unique=>true, :name=>"index_routes_on_namespace_id", :algorithm=>:concurrently})
   -> 0.0072s
-- execute("RESET statement_timeout")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:routes)
   -> 0.0019s
-- transaction_open?()
   -> 0.0000s
-- execute("LOCK TABLE namespaces, routes IN SHARE ROW EXCLUSIVE MODE")
   -> 0.0004s
-- execute("ALTER TABLE routes\nADD CONSTRAINT fk_679ff8213d\nFOREIGN KEY (namespace_id)\nREFERENCES namespaces (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0016s
-- execute("ALTER TABLE routes VALIDATE CONSTRAINT fk_679ff8213d;")
   -> 0.0027s
== 20220107091629 AddRouteNamespaceIndex: migrated (0.0230s) ==================


$ rake db:rollback
== 20220107091629 AddRouteNamespaceIndex: reverting ===========================
-- transaction_open?()
   -> 0.0000s
-- remove_foreign_key(:routes, {:column=>:namespace_id})
   -> 0.0030s
-- transaction_open?()
   -> 0.0000s
-- indexes(:routes)
   -> 0.0024s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- remove_index(:routes, {:algorithm=>:concurrently, :name=>"index_routes_on_namespace_id"})
   -> 0.0038s
-- execute("RESET statement_timeout")
   -> 0.0003s
== 20220107091629 AddRouteNamespaceIndex: reverted (0.0189s) ==================

== 20220105121325 AddRouteNamespaceReference: reverting =======================
-- column_exists?(:routes, :namespace_id)
   -> 0.0008s
-- remove_column(:routes, :namespace_id)
   -> 0.0006s
== 20220105121325 AddRouteNamespaceReference: reverted (0.0015s) ==============

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