Granular link checking

This adds a feature that allows editors to check their documents for the presence of broken links (i.e. links to pages that do not return a successful response). For now, broken link checking is manually instigated by users via a new panel on the right of the page:

![screen shot 2014-06-19 at 17 36 41](https://cloud.githubusercontent.com/assets/3687/3330382/2bc50c32-f7d0-11e3-9fa3-5d6e2d0b874a.png)

Users see a message whilst the report is running:

![screen shot 2014-06-19 at 17 38 58](https://cloud.githubusercontent.com/assets/3687/3330407/6209a154-f7d0-11e3-8ae1-7d7b7f0d63dc.png)

This automatically updates to show the results when the links have been checked:

![screen shot 2014-06-19 at 17 41 52](https://cloud.githubusercontent.com/assets/3687/3330429/a487cc18-f7d0-11e3-80af-69dd6f2eb827.png)

The report is automatically re-run if a document with broken links is edited. When the document has the all clear, they see this:

![screen shot 2014-06-19 at 17 43 19](https://cloud.githubusercontent.com/assets/3687/3330450/d58d0684-f7d0-11e3-8546-a3f4f2bbecb4.png)



https://www.agileplannerapp.com/boards/173808/cards/3358