Resolve "Recover reverted fixes to Merge Request refactor regressions"

What does this MR do?

Recovers the fixes we had to revert to remove any possible cause for the performance degradation that caused 11.1.3 to be aborted.

Recovered by this MR

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20642 Fix rendering context lines on MR diffs
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20569 Fix autosave issues for MR discussions
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20434 Resolve ""Jump to first/next unresolved discussion" jumps to resolved discussions"
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20779 Improving performance of posting a comment by reducing memory + CPU (currently removed from this branch see comment below.)

Please check when each of these have been reviewed thoroughly. See next point.

Are there points in the code the reviewer needs to double check?

Do deep performance profiling on:

1. rendering the Changes tab on small as well as large (+5000 changes) MRs.
2. posting notes
3. resolving discussions

Rule of thumb: this cannot be worse than what is in master. It should even be quicker when posting comment since we are recovering !20779 (merged).

Why was this MR needed?

We had to remove the fixes to no block 11.2 RC1.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them I think we should not squash this, seeing that each commit is individual bundle of work easily revertable.
• Internationalization required/considered

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49242 and https://gitlab.com/gitlab-org/gitlab-ce/issues/49343 and https://gitlab.com/gitlab-org/gitlab-ce/issues/48876 and https://gitlab.com/gitlab-org/gitlab-ce/issues/48877 and #48817 (closed) and #48602 (closed) and #49843 (closed)