Fix disabled layout.html override for users.

Turns out Sphinx only lets you override an HTML template once. Me
overriding it here for the banner prevented users from overriding it
themselves (for adding css files/etc).

Using an alternative approach to inserting the banner HTML in the body
variable: Jinja2 context['body'].

sphinxcontrib/versioning/sphinx_.py

@@ -112,6 +112,11 @@ def html_page_context(cls, app, pagename, templatename, context, doctree):
         context['vhasdoc'] = versions.vhasdoc
         context['vpathto'] = versions.vpathto
 
+        # Insert banner into body.
+        if cls.SHOW_BANNER and 'body' in context:
+            parsed = app.builder.templates.render('banner.html', context)
+            context['body'] = parsed + context['body']
+
 
 def setup(app):
     """Called by Sphinx during phase 0 (initialization).

tests/test_sphinx/test_build.py

@@ -149,6 +149,30 @@ def test_versions_override(tmpdir, local_docs):
     assert '<li>BitBucket: feature</li>' in contents
 
 
+def test_layout_override(tmpdir, local_docs):
+    """Verify users can still override layout.html.
+
+    :param tmpdir: pytest fixture.
+    :param local_docs: conftest fixture.
+    """
+    versions = Versions([('', 'master', 'heads', 1, 'conf.py')])
+
+    local_docs.join('conf.py').write(
+        'templates_path = ["_templates"]\n'
+    )
+    local_docs.ensure('_templates', 'layout.html').write(
+        '{% extends "!layout.html" %}\n'
+        '{% block extrahead %}\n'
+        '<!-- Hidden Message -->\n'
+        '{% endblock %}\n'
+    )
+
+    target = tmpdir.ensure_dir('target_master')
+    build(str(local_docs), str(target), versions, 'master', True)
+    contents = target.join('contents.html').read()
+    assert '<!-- Hidden Message -->' in contents
+
+
 def test_subdirs(tmpdir, local_docs, urls):
     """Make sure relative URLs in `versions` works with RST files in subdirectories.