Add uuid column to the security_findings table

Adds a new column called uuid into the security_findings table

Issue: 277325

Created a migration to add the column uuid to security_findings.

Database review

rake db:migrate

== 20201202025644 AddColumnToSecurityFindings: migrating ======================
-- add_column(:security_findings, :uuid, :uuid)
   -> 0.0051s
== 20201202025644 AddColumnToSecurityFindings: migrated (0.0051s) =============

== 20201202025937 AddIndexToSecurityFindingsUuid: migrating ===================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:security_findings, :uuid, {:name=>"index_security_findings_on_uuid", :unique=>true, :algorithm=>:concurrently})
   -> 0.0034s
-- add_index(:security_findings, :uuid, {:name=>"index_security_findings_on_uuid", :unique=>true, :algorithm=>:concurrently})
   -> 0.0196s
== 20201202025937 AddIndexToSecurityFindingsUuid: migrated (0.0235s) ==========

rake db:migrate:down

== 20201202025937 AddIndexToSecurityFindingsUuid: reverting ===================
-- transaction_open?()
   -> 0.0000s
-- indexes(:security_findings)
   -> 0.0040s
-- remove_index(:security_findings, {:algorithm=>:concurrently, :name=>"index_security_findings_on_uuid"})
   -> 0.0026s
== 20201202025937 AddIndexToSecurityFindingsUuid: reverted (0.0070s) ==========

== 20201202025644 AddColumnToSecurityFindings: reverting ======================
-- remove_column(:security_findings, :uuid, :uuid)
   -> 0.0008s
== 20201202025644 AddColumnToSecurityFindings: reverted (0.0028s) =============

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team