Populate remaning dismissal information for vulnerabilities

What does this MR do?

This post-deployment migration is required to run according to docs.

Related to #262112 (closed)

Database migration

rake db:migrate:up

== 20201124122817 PopulateRemainingMissingDismissalInformationForVulnerabilities: migrating
== 20201124122817 PopulateRemainingMissingDismissalInformationForVulnerabilities: migrated (0.0364s)

rake db:migrate:down

== 20201124122817 PopulateRemainingMissingDismissalInformationForVulnerabilities: reverting
== 20201124122817 PopulateRemainingMissingDismissalInformationForVulnerabilities: reverted (0.0000s)

Timing

There are currently 510 records that will be returned by the Vulnerability.broken scope. Those records will not be able to migrated as they have other issues and we will remove those ones from production with a follow-up issue. But we need this migration for self-hosted instances as well.

I've tested the related logic on console without the side-effects and it took ~4 seconds.

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