Resolve "De-duplicate diff lines from backend"

What does this MR do?

Performance improvement for MR Diffs. Remove parallel_diff_lines from diff_file entity and compute on frontend. Also saves a loop over the diff lines for trimFirstCharOfLineContent.

Convert parallelize function from parallel_diff.rb to utils.js

Sample MR with 100 changed files. Reduced http://localhost:3000/Commit451/lab-coat/merge_requests/3/diffs.json from 4.9MB -> 2MB

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50290

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.