Ensure links to external websites contain rel="nofollow"

[dhedlund]

This ticket should follow or be done at the same time as issue #25. When someone adds a link to one of the following fields, those links should get the rel="nofollow" property added to them:

• Event#url
• Event#description
• Event#venue_details
• Venue#url
• Venue#description
• Venue#access_notes
• Source#url

This should be done server-side as the primary user of this information is search engines; we want to discourage the site from being used to increase another site's pagerank via spamming. See: https://support.google.com/webmasters/answer/96569?hl=en

The easiest solution I can find involves using Nokogiri to parse the XHTML and then add the property to each link. See the following for an example:
tribut/juvia@6bfadb0

There's only one field I can think of where we might not want to add the rel="nofollow" property to which is Organization#url. It would be nice for search engines to pick up the relationship ActivateHub and the organizations we try and help serve.