Preserve newlines in descriptions and comments

This formats the incident description using Rails' simple_format helper,
which converts newlines into paragraphs and line break tags.

Comments are rendered in JavaScript, so they're handled separately using
string substitution. Hopefully this will be superseded by Markdown
support in future, but this will make things more usable until then.
Merge pull request #14 from intercom/elk/preserve_newlines

Preserve newlines in descriptions and comments