Improve Vulnerability Tracking: Add fingerprints table

This MR is the database portion of the proof-of-concept MR: !45339 (closed) and is part of the epic https://gitlab.com/groups/gitlab-org/-/epics/4690 - Improve Vulnerability Tracking: MVP: Scope+Offset

What does this MR do?

This MR adds a vulnerability_tracking_fingerprints table that enables each vulnerability_occurrence to have multiple vulnerability tracking fingerprints.

Please see the description of the backend implementation MR for an overview of how this fits in with backend code changes in a separate merge request.

Note

@d0c-s4vage did the initial work on this MR and others as part of the epic, but @rossfuhrman will be pushing this through the review process.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) -- Not required, unused DB table only
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Database Migration Output

 lptp [ gitlab ]: bundle exec rake db:migrate:up VERSION=20201108134919 RAILS_ENV=development
== 20201108134919 AddTrackingFingerprintTable: migrating ======================
-- create_table(:vulnerability_tracking_fingerprints)
   -> 0.0335s
== 20201108134919 AddTrackingFingerprintTable: migrated (0.0356s) =============

 lptp [ gitlab ]: bundle exec rake db:migrate:down VERSION=20201108134919 RAILS_ENV=development
== 20201108134919 AddTrackingFingerprintTable: reverting ======================
-- drop_table(:vulnerability_tracking_fingerprints)
   -> 0.0011s
== 20201108134919 AddTrackingFingerprintTable: reverted (0.0030s) =============

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