helpers: in urlify_text, use <br> for newlines in pre-formatted text so it can be cut'n'pasted correctly

Before, when copying the preformatted nice looking ASCII-artsy text in a PR or
changeset description from a browser to a text editor, it would be pasted as
text without newline. Simply put, copy/paste ignores that it is inside a <pre>
/ white-space:pre-wrap section.

Instead, translate newlines to <br> which always translates to a newline when
pasted.