Add Markdown to the app root
Add jinja2_markup extension to the app root
Add markdown tag to jinja2
Remove br substitution

This is now done using the Jinja2 Markdown extension
Content inside {% markdown %} content! {% endmarkdown %} tags is
interpreted as Markdown.
Remove ugly hack

This page.render() call used to store in a render_str_ variable the
result of substituting all newlines in the page.content with <br> tags.
Ugly.
Now we use Markdown!
Update templates to use markdown tag