Speed up expanding/collapsing diff files

What does this MR do?

Halve the time it takes to collapse diff files.

Basically just inlining one component inside another one.

Move diff_cell_contents into the wrapper component to save excessive Vue instances on the page. Since there are multiple diff_cells for each line of a diff file, there were thousands of these on large files.

Also move spec to jest

Screenshots

Perf trace showing expanding/collapsing a 440 line diff file twice.

Before (6s)	After (2.9s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry - don't think necessary?
• Code review guidelines
• Merge request performance guidelines
• Style guides

Assists #26693 (closed)