Add RAW file format which is used to store security reports

What does this MR do?

Requires: gitlab-runner!1057 (merged)

- This introduces and uses `:raw` format for all legacy reports,
  the ones that do not have yet proper parsers on Backend
- Raw format is needed to make Frontend be able to parse reports,
  without the need of decompressing,
- This also extends fixtures to seed security reports with database,
  even though parser code is part of EE

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7996

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides