Add section to approval rule name index

What does this MR do?

Add section to name/rule_type index on ApproveRuleMergeRequests

== 20200526231421 UpdateIndexApprovalRuleNameForCodeOwnersRuleType: migrating =
-- indexes(:approval_merge_request_rules)
   -> 0.0057s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:approval_merge_request_rules, [:merge_request_id, :name, :section], {:unique=>true, :where=>"rule_type = 2", :name=>"index_approval_rule_name_for_sectional_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0025s
-- add_index(:approval_merge_request_rules, [:merge_request_id, :name, :section], {:unique=>true, :where=>"rule_type = 2", :name=>"index_approval_rule_name_for_sectional_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0028s
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_merge_request_rules)
   -> 0.0019s
-- remove_index(:approval_merge_request_rules, {:algorithm=>:concurrently, :name=>"index_approval_rule_name_for_code_owners_rule_type"})
   -> 0.0023s
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_merge_request_rules)
   -> 0.0019s
-- remove_index(:approval_merge_request_rules, {:algorithm=>:concurrently, :name=>"approval_rule_name_index_for_code_owners"})
   -> 0.0007s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:approval_merge_request_rules, [:merge_request_id, :name], {:unique=>true, :where=>"rule_type = 2 AND section IS NULL", :name=>"index_approval_rule_name_for_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0021s
-- add_index(:approval_merge_request_rules, [:merge_request_id, :name], {:unique=>true, :where=>"rule_type = 2 AND section IS NULL", :name=>"index_approval_rule_name_for_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0033s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:approval_merge_request_rules, [:merge_request_id, :code_owner, :name], {:unique=>true, :where=>"code_owner = true AND section IS NULL", :name=>"approval_rule_name_index_for_code_owners", :algorithm=>:concurrently})
   -> 0.0024s
-- add_index(:approval_merge_request_rules, [:merge_request_id, :code_owner, :name], {:unique=>true, :where=>"code_owner = true AND section IS NULL", :name=>"approval_rule_name_index_for_code_owners", :algorithm=>:concurrently})
   -> 0.0019s
== 20200526231421 UpdateIndexApprovalRuleNameForCodeOwnersRuleType: migrated (0.0299s)

See also !32723 (closed)

Related #215194 (closed)