Add artifact file page that uses the blob viewer

app/assets/javascripts/dispatcher.js

app/controllers/projects/artifacts_controller.rb

app/helpers/blob_helper.rb

app/helpers/gitlab_routing_helper.rb

app/models/ci/artifact_blob.rb

app/views/projects/artifacts/_tree_file.html.haml

app/views/projects/artifacts/file.html.haml

changelogs/unreleased/dm-artifact-blob-viewer.yml

config/routes/project.rb

features/project/builds/artifacts.feature

features/steps/project/builds/artifacts.rb

lib/gitlab/ci/build/artifacts/metadata/entry.rb

spec/controllers/projects/artifacts_controller_spec.rb

spec/features/projects/artifacts/file_spec.rb

spec/lib/gitlab/ci/build/artifacts/metadata/entry_spec.rb

spec/models/ci/artifact_blob_spec.rb