Merge branch 'dm-dependency-linker-gemfile' into 'master'

app/assets/stylesheets/highlight/dark.scss

app/assets/stylesheets/highlight/monokai.scss

app/assets/stylesheets/highlight/solarized_dark.scss

app/assets/stylesheets/highlight/solarized_light.scss

app/assets/stylesheets/highlight/white.scss

app/services/system_note_service.rb

changelogs/unreleased/dm-dependency-linker-gemfile.yml

lib/gitlab/dependency_linker.rb

lib/gitlab/dependency_linker/base_linker.rb

lib/gitlab/dependency_linker/gemfile_linker.rb

lib/gitlab/diff/inline_diff_markdown_marker.rb

lib/gitlab/diff/inline_diff_marker.rb

lib/gitlab/highlight.rb

lib/gitlab/string_range_marker.rb

lib/gitlab/string_regex_marker.rb

spec/helpers/diff_helper_spec.rb

spec/lib/gitlab/dependency_linker/gemfile_linker_spec.rb

spec/lib/gitlab/dependency_linker_spec.rb

spec/lib/gitlab/diff/highlight_spec.rb

spec/lib/gitlab/diff/inline_diff_markdown_marker_spec.rb