Add unfold links for Side-by-Side view

CHANGELOG

app/assets/javascripts/diff.js

app/controllers/projects/blob_controller.rb

app/helpers/diff_helper.rb

app/views/projects/blob/diff.html.haml

app/views/projects/diffs/_content.html.haml

app/views/projects/diffs/_diffs.html.haml

app/views/projects/diffs/_line.html.haml

app/views/projects/diffs/_match_line.html.haml

app/views/projects/diffs/_parallel_view.html.haml

app/views/projects/diffs/_text_file.html.haml

app/views/projects/merge_requests/_show.html.haml

features/project/merge_requests.feature

features/steps/project/merge_requests.rb

spec/helpers/diff_helper_spec.rb