When enabling the `single_mr_diff_view` feature flag, the backend will
only render one kind of diff: inline or parallel. The feature is
disabled for now until the frontend has been updated.

To switch the view, the frontend can pass a `view` param containing
either `inline` or `parallel`. That will get written to a cookie. The
response for diff files will then contain either
`highlighted_diff_lines` or `parallel_diff_lines` but no longer both.