Considerably improve the loading time on merge request's discussion page

What does this MR do?

1. Caches and uses highlighted diff lines on merge request discussions
2. Improves the main diff line lookup for each discussion

Profilling results (locally)

Before

Consistent response time (55 diff discussions, 10 resolved):

Completed 200 OK in 7461ms (Views: 166.8ms | ActiveRecord: 26.6ms)

After

Consistent response time (55 diff discussions, 10 resolved):

Completed 200 OK in 4398ms (Views: 168.0ms | ActiveRecord: 26.5ms)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52950

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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