Add VulnerabilityStateTransition model

This model will be responsible for storing state transitions of a Vulnerability.

#325700 (closed)

Migration up

rake db:migrate:up:main VERSION=20220517182529

== 20220517182529 CreateVulnerabilityStateTransition: migrating ===============
-- create_table(:vulnerability_state_transitions, {})
   -> 0.0151s
== 20220517182529 CreateVulnerabilityStateTransition: migrated (0.0152s) ======

Migration down

rake db:migrate:down:main VERSION=20220517182529

== 20220517182529 CreateVulnerabilityStateTransition: reverting ===============
-- drop_table(:vulnerability_state_transitions)
   -> 0.0060s
== 20220517182529 CreateVulnerabilityStateTransition: reverted (0.0061s) ======

app/models/vulnerability_state_transition.rb

+# frozen_string_literal: true
+
+class VulnerabilityStateTransition < ApplicationRecord
+  belongs_to :vulnerability
+end
+
+VulnerabilityStateTransition.prepend_mod