Add 'View on [env]' link to blobs and individual files in diffs

app/controllers/projects/blob_controller.rb

app/controllers/projects/commit_controller.rb

app/controllers/projects/compare_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/helpers/commits_helper.rb

app/models/ci/pipeline.rb

app/models/environment.rb

app/models/merge_request.rb

app/models/project.rb

app/models/repository.rb

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

app/views/projects/commit/show.html.haml

app/views/projects/compare/show.html.haml

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

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

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

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

changelogs/unreleased/route-map.yml