Add `rule_type` to indexes explicitly

This means that we're enforcing uniqueness of a rule name per approval
merge request type on MySQL, since partial indexes are not supported
there.

On postgres, `rule_type` was already part of the index, but as the
index is partial it would not apply for rules that are not code owner
rules.

[run mysql]

ee/db/migrate/20190528173628_add_index_for_code_owner_rule_type_on_approval_merge_request_rules.rb

@@ -26,7 +26,7 @@ class AddIndexForCodeOwnerRuleTypeOnApprovalMergeRequestRules < ActiveRecord::Mi
     # Ensure only 1 code_owner rule per merge_request
     add_concurrent_index(
       :approval_merge_request_rules,
-      [:merge_request_id, :name],
+      [:merge_request_id, :rule_type, :name],
       unique: true,
       where: "rule_type = #{ApprovalMergeRequestRule.rule_types[:code_owner]}",
       name: INDEX_CODE_OWNERS_RULES_UNIQUENESS_NAME
@@ -35,7 +35,7 @@ class AddIndexForCodeOwnerRuleTypeOnApprovalMergeRequestRules < ActiveRecord::Mi
     # Support lookups for all code_owner rules per merge_request
     add_concurrent_index(
       :approval_merge_request_rules,
-      [:merge_request_id],
+      [:merge_request_id, :rule_type],
       where: "rule_type = #{ApprovalMergeRequestRule.rule_types[:code_owner]}",
       name: INDEX_CODE_OWNERS_RULES_QUERY_NAME
     )