Allow expanding context lines on MR Discussion tab
Part A
It would be nice to enhance our view of the MR Discussion tab so that we can expand the diff context, just like we can on the Changes tab.
For instance, here:
The reason this is a tricky change needs the if
that this is in, which isn't shown in that context. On the Changes tab, we can expand this by using the ellipses:
We should be able to do that on the discussion tab, too.
Part B
Of course, if we do it that way, we can only expand upwards because expanding downwards would be confusing. However, we could add some context lines below the note on the Discussion tab, as in the mockups from https://gitlab.com/gitlab-org/gitlab-ce/issues/23672. Then we could add the ellipses at the top of the before context, and the bottom of the after context. And this could be done in any order.