Add severity_levels column into approval_project_rules table

What does this MR do?

Add severity_levels column into approval_project_rules table.

Follow-up MR with validation and usage: !67064 (merged)

Related issue: #335333 (closed)

bundle exec rails db:migrate VERSION=20210726202748

== 20210726202748 AddVulnerabilitySeveritiesIntoApprovalProjectRules: migrating
-- add_column(:approval_project_rules, :severity_levels, :text, {:array=>true})
   -> 0.0026s
== 20210726202748 AddVulnerabilitySeveritiesIntoApprovalProjectRules: migrated (0.0027s)

bundle exec rails db:rollback VERSION=20210726202748

== 20210726202748 AddVulnerabilitySeveritiesIntoApprovalProjectRules: reverting
-- remove_column(:approval_project_rules, :severity_levels)
   -> 0.0022s
== 20210726202748 AddVulnerabilitySeveritiesIntoApprovalProjectRules: reverted (0.0023s)

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How to setup and validate locally (strongly suggested)

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