WIP: Add support for CI build annotations

What does this MR do?

This adds support for CI build annotations, as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/51641. A more in-depth description will be added once the changes mature.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51641

TODO

• Add mechanism for caching rendered Markdown in Redis.
  • This should be implemented in such a way that N+1 requests are not possible. This would be done using Redis pipelining, and raising errors if the data is not eager loaded (= N+1 is impossible).
  • This somehow should reuse some of the important logic from the CacheMarkdownField concern. This may just require some copy-pasting for the time being. We also don't need to support multiple Markdown engines.
• Create Sidekiq worker for processing the reports
• Schedule Sidekiq worker when the build state transitions to a finished state, like security reports do for pipelines
• Create service that takes a CI build, and parses it into a database row
  • Ensure that we only process artifacts below 10MB in size
• Implement basic UI, which initially would just be a separate "Annotations" tab in an MR
• Tests for the Sidekiq worker

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer