Add member_namespace_id column to members table

What does this MR do and why?

This reference will serve as a replacement of the existing source polymorphic reference - we will migrate to using member_namespace

Database

> rake db:migrate
== 20220112232037 AddMemberNamespaceReference: migrating ======================
-- column_exists?(:members, :member_namespace_id)
   -> 0.0060s
-- add_column(:members, :member_namespace_id, :bigint)
   -> 0.0023s
== 20220112232037 AddMemberNamespaceReference: migrated (0.0085s) =============

== 20220112232605 AddMemberNamespaceIndex: migrating ==========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:members, :member_namespace_id, {:unique=>false, :name=>"index_members_on_member_namespace_id", :algorithm=>:concurrently})
   -> 0.0088s
-- execute("SET statement_timeout TO 0")
   -> 0.0010s
-- add_index(:members, :member_namespace_id, {:unique=>false, :name=>"index_members_on_member_namespace_id", :algorithm=>:concurrently})
   -> 0.0053s
-- execute("RESET statement_timeout")
   -> 0.0007s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:members)
   -> 0.0051s
-- transaction_open?()
   -> 0.0000s
-- execute("LOCK TABLE namespaces, members IN SHARE ROW EXCLUSIVE MODE")
   -> 0.0014s
-- execute("ALTER TABLE members\nADD CONSTRAINT fk_aa82dcc1c6\nFOREIGN KEY (member_namespace_id)\nREFERENCES namespaces (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0045s
-- execute("ALTER TABLE members VALIDATE CONSTRAINT fk_aa82dcc1c6;")
   -> 0.0061s
== 20220112232605 AddMemberNamespaceIndex: migrated (0.0465s) =================

> rake db:rollback
== 20220112232605 AddMemberNamespaceIndex: reverting ==========================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:members)
   -> 0.0036s
-- remove_foreign_key(:members, {:column=>:member_namespace_id})
   -> 0.0039s
-- transaction_open?()
   -> 0.0000s
-- indexes(:members)
   -> 0.0061s
-- execute("SET statement_timeout TO 0")
   -> 0.0009s
-- remove_index(:members, {:algorithm=>:concurrently, :name=>"index_members_on_member_namespace_id"})
   -> 0.0055s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20220112232605 AddMemberNamespaceIndex: reverted (0.0397s) =================

> rake db:rollback
== 20220112232037 AddMemberNamespaceReference: reverting ======================
-- column_exists?(:members, :member_namespace_id)
   -> 0.0028s
-- remove_column(:members, :member_namespace_id)
   -> 0.0023s
== 20220112232037 AddMemberNamespaceReference: reverted (0.0052s) =============

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Related to #349573 (closed)