Make our custom HTML page template work with Sphinx 1.6.1+

[tjanez]

Building documentation with the latest version of Sphinx (1.6.1) fails with:

$ python setup.py build_sphinx --fresh-env --warning-is-error
running build_sphinx
Running Sphinx v1.6.1
loading intersphinx inventory from https://docs.python.org/3/objects.inv...
loading intersphinx inventory from https://resolwe.readthedocs.io/en/latest/objects.inv...
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 10 source files that are out of date
updating environment: 10 added, 0 changed, 0 removed
reading sources... [100%] ref                                                                                                               
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [ 10%] CHANGELOG                                                                                                          
Exception occurred:
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx_rtd_theme/layout.html", line 45, in top-level template code
    {% for cssfile in css_files %}
TypeError: 'NoneType' object is not iterable
The full traceback has been saved in /tmp/sphinx-err-453cydcj.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

Full traceback in /tmp/sphinx-err-453cydcj.log is:

Traceback (most recent call last):
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/setup_command.py", line 192, in run
    app.build(force_all=self.all_files)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/application.py", line 338, in build
    self.builder.build_update()
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/__init__.py", line 328, in build_update
    'out of date' % len(to_build))
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/__init__.py", line 394, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/__init__.py", line 431, in write
    self._write_serial(sorted(docnames))
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/__init__.py", line 440, in _write_serial
    self.write_doc(docname, doctree)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/html.py", line 556, in write_doc
    self.handle_page(docname, ctx, event_arg=doctree)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/builders/html.py", line 940, in handle_page
    output = self.templates.render(templatename, ctx)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/jinja2glue.py", line 176, in render
    return self.environment.get_template(template).render(context)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "/home/tadej/Genialis/resolwe-bio/docs/_templates/page.html", line 3, in top-level template code
    {% set css_files = css_files + ["_static/css/custom.css"] %}
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
    {%- extends "layout.html" %}
  File "/home/tadej/.virtualenvs/resolwe-bio/lib/python3.5/site-packages/sphinx_rtd_theme/layout.html", line 45, in top-level template code
    {% for cssfile in css_files %}
TypeError: 'NoneType' object is not iterable

It looks like something is wrong in our docs/_templates/page.html file on line:

{% set css_files = css_files + ["_static/css/custom.css"] %}