Allow suggesting single line changes in diffs

What does this MR do?

Allow suggesting single line changes in MR diffs.

• Creates a suggestions DB table
• Introduces a /api/v4/suggestions/:id/apply API
• Allows users using ```suggestion syntax to suggest changes in MRs
• These changes are presented as a diff table in the created note
• Users with authorization can click Apply suggestion and this will add commit to the source branch
• Outdated suggestions (the commented line changes) are not appliable

Feature flag

• If disabled; the syntax will stop generating the suggestion table
• If disabled; existing suggestion tables will be presented (they're persisted in note_html already)
• If disabled; users won't be able to apply the already existing suggestions
• If disabled; the "Insert suggestion" button in the toolbar will not be presented

Important points

• Documentation will be added in a separate MR
• Port to EE
• [EE] Draft notes (review comments) doesn't support the diff rendering, but the diff appear after its submission (see https://gitlab.slack.com/archives/CE3DX19EX/p1543616087001000)
• Highlighting is slightly limited, we're currently not presenting syntax highlighting
• Review app: https://gitlab-review-osw-sugges-j15ey3.ce.gitlab-review.app/root/test-project/merge_requests/1/diffs

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/18008

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.