Add a temporary index for Member.state

What does this MR do and why?

Adds a temporary index against Member.state so that we can more efficiently run a data migration to mass update member states (!76653 (merged)).

Further information around the reasoning behind this can be found here: !76750 (comment 790086191)

This index takes the data migration from 117s to 2ms (see thread linked above for more info)

Migrate up

rake db:migrate:up VERSION=20211223125921

== 20211223125921 AddTempIndexToMembersState: migrating =======================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:members, :state, {:name=>"tmp_index_members_on_state", :where=>"state = 2", :algorithm=>:concurrently})
   -> 0.0048s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:members, :state, {:name=>"tmp_index_members_on_state", :where=>"state = 2", :algorithm=>:concurrently})
   -> 0.0115s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20211223125921 AddTempIndexToMembersState: migrated (0.0199s) ==============

Migrate down

 rake db:migrate:down VERSION=20211223125921
== 20211223125921 AddTempIndexToMembersState: reverting =======================
-- transaction_open?()
   -> 0.0000s
-- indexes(:members)
   -> 0.0064s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- remove_index(:members, {:algorithm=>:concurrently, :name=>"tmp_index_members_on_state"})
   -> 0.0060s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20211223125921 AddTempIndexToMembersState: reverted (0.0162s) ==============

  CREATE INDEX tmp_index_members_on_state ON members USING btree (state) WHERE (state = 2)

Time to apply the index in #database-lab:

Cold cache: 1.977 mins (internal slack)

Warm cache: 11.847s (internal slack)

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