Fix mistune imports for mistune >= 2.0.0
hyperkitty/lib/renderer.py:
With mistune 2.0.0 the escape_url() and escape_html() are moved to
mistune.util.
Fixes #395 (closed)
hyperkitty/lib/renderer.py:
With mistune 2.0.0 the escape_url() and escape_html() are moved to
mistune.util.
Fixes #395 (closed)