WIP: Resolves "Allow expanding context lines on MR Discussion tab"

What does this MR do?

• Part A: expand diff above nested discussion
• Part B: display some context lines below nested discussion and expand diff below (hopefully)

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

Handling of plain via data-* and UploadForm.

cc @DouweM @smcgivern

Why was this MR needed?

Assymetric functionality between Discussion and Changes tabs.

Screenshots (if relevant)

Before:

After (ellipsis):

After (expanded):

After (middle):

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24138 (moved)