Add form fields for adding related external links.
Tweak controller to not create blank related_link on create
Add nested form gem
Include nested forms JS
Add external links fields to the artefact form

Uses the nested_forms gem to do it.
Bump content models gem to 5.5.0
Improve external links testing

Add poltergeist gem to run tests with JS and set up Capybara
accordingly.

Now don't show a blank new external link field on edit, but just the
button.
Explicitly set the capybara version
Abstract creation of artefact back into filter
Use row-fluid so it works when the browser width changes
Fix indentation in controller
Update the integration test to not rely on the presentation class name
Merge remote-tracking branch 'origin/related_external_links'