preserve linebreaks in textarea submissions

This is a minimal fix for http://jira.wnyc.net/browse/CROWD-35, "text box and text area submissions do not preserve formatting or line breaks".

It modifies the template partial to replace newlines with HTML BR tags on output.

Supporting other formatting would require that we switch from using Django's built-in form sanitizing to a custom method; we should probably also consider supporting an extended textarea that allows WYSIWYG / Markdown / or similar.