Make the three MR Diff line expansion links always align regardless of which ones are showing

Problem to solve

Diff expansion lines might fall under these categories:

• No lines above
• No lines below
• Lines above and below

In each of these instances, we hide the links that don't apply, causing them to misalign.

User experience goal

Easily discern which one I want to click.

Proposal

[Full screenshot(/uploads/b42767b66290d06e092bd29c08398972/Screenshot_2020-11-17_at_14.56.26.png)

Links / references