Only show 'Last updated on ...' when last_updated defined

[hugovk]

Fixes #182.

We were always showing it:

{% trans last_updated=last_updated|e %}Last updated on {{ last_updated }}.{% endtrans %}

Instead, we should check the variable first:

{%- if last_updated %}
  {% trans last_updated=last_updated|e %}Last updated on {{ last_updated }}.{% endtrans %}
{%- endif %}

This is also what Sphinx does:

https://github.com/sphinx-doc/sphinx/blob/2e05fd2ffe70b2cfd48bdb9e35eecac43b46637d/sphinx/themes/basic/layout.html#L211-L213

Also add some newlines around other if blocks to make them clearer.