Change documents sections

PT Story: https://www.pivotaltracker.com/story/show/72330554
Heroku app: http://damp-lowlands-2351.herokuapp.com/ (you have to be logged in to seeing the "Change section" button in a document. 

Brief description of the changes: 
- added button for changing the section in the documents show page
- using pop-up window for choosing the new section (as suggested by @YaroSpace here: http://agileventures.org/projects/websiteone/documents/lofi-documents-management) 
- It won't be very clear in heroku's instance because of the funky names, but the sections available for choosing are the ones amongst this project, as well the current document isn't available for choosing. 
- It also doesn't show the children of the current document in order to avoid loopholes. 
I couldn't find the proper syntax for that. The one pointed by you does not show anything. (I think find returns just one result though)
When trying in the console: Document.find(porject: project) returns 
can't cast Hast to Integer

As well, we need documents with id different from the current document's one.