Add new column `finding_uuid` into `vulnerability_feedback` table

What does this MR do?

This MR adds a vulnerability finding UUID field to the vulnerability feedback table, to connect the feedback to the finding. This is part of an effort to replace the project_fingerprint with a unique ID. This MR resolves #277130 (closed).

Additional work will be done in issues #277133 (closed) and #277134 (closed).

Migration Output

Migrate Up

== 20201201163227 AddFindingUuidToVulnerabilityFeedback: migrating ============
-- add_column(:vulnerability_feedback, :finding_uuid, :uuid)
   -> 0.0012s
== 20201201163227 AddFindingUuidToVulnerabilityFeedback: migrated (0.0013s) ===

Rollback

== 20201201163227 AddFindingUuidToVulnerabilityFeedback: reverting ============
-- remove_column(:vulnerability_feedback, :finding_uuid, :uuid)
   -> 0.0011s
== 20201201163227 AddFindingUuidToVulnerabilityFeedback: reverted (0.0045s) ===

Screenshots (strongly suggested)

N/A

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

This MR is low risk, as it only adds a column to the database.

• [-] 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

Related to #277130 (closed)