Fix 18858: Link to docs built by dartdocs.org

Fix http://dartbug.com/18858
additionnal column for documentation and default documentation
fix typo
move dartdocs link to app/models/package.py
move dartdocs link into app/models/package_version.py
dartdocs_url on versions tab
fix dartdocs_url
Truncate long sidebar URLs.
Use the pubspec's documentation link in the version list if possible.
Merge branch 'dartdocs-link'