Convert newlines to <br /> in messages to source's

Hopefully fixes #521 by converting newline character to <br /> tags in messages before being displayed on the /lookup page.
Sorry, I still don't understand why we need this code. `jinja2.escape` already returns an escaped `Markup` object, see the [docs](http://jinja.pocoo.org/docs/dev/api/#jinja2.Markup):

> If you want object passed being always treated as unsafe you can use the escape() classmethod to create a Markup object:

So isn't this `if` statement redundant? Or am I missing something? Seems like you don't need this to be an `evalcontextfilter` at all, could just be a regular filter.