Add backend for merge conflicts reading

app/assets/javascripts/merge_conflict_resolver.js.coffee

app/assets/javascripts/merge_conflicts_data.coffee

app/controllers/projects/merge_requests_controller.rb

app/views/projects/merge_requests/show/_conflicts.html.haml

lib/gitlab/conflict/file.rb

lib/gitlab/conflict/parser.rb

lib/gitlab/diff/line.rb

spec/lib/gitlab/conflict/file_spec.rb

spec/lib/gitlab/conflict/parser_spec.rb

spec/support/test_env.rb