Prepares suggestion implementation for multi-line support

What does this MR do?

It implements the backend code required for creating multi-line suggestions in the DB. This adds the groundwork for reusing the same services for both persistence and Preview request. Additonally, it handles the outdating strategy which will be required to outdate multi-line suggestions.

Part of:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26069
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26057
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26107

Integration MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25211

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/53310

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