Add rule_index to security_orchestration_policy_rule_schedules

What does this MR do?

Addresses: #337537 (closed)

This MR adds rule_index column to security_orchestration_policy_rule_schedules table. This column will be used to store the index of rules value that is part of the Security Policy Schema

bundle exec rails db:migrate VERSION=20210805085706

== 20210805085706 AddRuleIndexToSecurityOrchestrationPolicyRuleSchedules: migrating 
-- add_column(:security_orchestration_policy_rule_schedules, :rule_index, :integer, {:null=>false, :default=>0})
   -> 0.0030s
== 20210805085706 AddRuleIndexToSecurityOrchestrationPolicyRuleSchedules: migrated (0.0125s) 

bundle exec rails db:rollback VERSION=20210805085706

== 20210805085706 AddRuleIndexToSecurityOrchestrationPolicyRuleSchedules: reverting 
-- remove_column(:security_orchestration_policy_rule_schedules, :rule_index)
   -> 0.0013s
== 20210805085706 AddRuleIndexToSecurityOrchestrationPolicyRuleSchedules: reverted (0.0076s) 

backend changes: !67607 (merged)

📓 Backward compatibility

This feature is still behind a feature flag and in production there are 0 rows for security_orchestration_policy_rule_schedules table. So this change will not have impact in the functionality.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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