Wrap nbconvert Markdown/Heading cells in live divs

Addresses https://github.com/ipython/ipython/issues/5206

This partially mimics the live IPython notebook layout, but is not a
complete solution.

IPython/nbconvert/templates/html/basic.tpl

@@ -1,173 +1,177 b''
 {%- extends 'display_priority.tpl' -%}
 
 
 {% block codecell %}
 <div class="cell border-box-sizing code_cell">
 {{ super() }}
 </div>
 {%- endblock codecell %}
 
 {% block input_group -%}
 <div class="input">
 {{ super() }}
 </div>
 {% endblock input_group %}
 
 {% block output_group %}
 <div class="output_wrapper">
 <div class="output">
 {{ super() }}
 </div>
 </div>
 {% endblock output_group %}
 
 {% block in_prompt -%}
 <div class="prompt input_prompt">
 In&nbsp;[{{ cell.prompt_number }}]:
 </div>
 {%- endblock in_prompt %}
 
 {% block empty_in_prompt -%}
 <div class="prompt input_prompt">
 </div>
 {%- endblock empty_in_prompt %}
 
 {# 
   output_prompt doesn't do anything in HTML,
   because there is a prompt div in each output area (see output block)
 #}
 {% block output_prompt %}
 {% endblock output_prompt %}
 
 {% block input %}
 <div class="input_area box-flex1">
 {{ cell.input | highlight2html(metadata=cell.metadata) }}
 </div>
 {%- endblock input %}
 
 {% block output %}
 <div class="output_area">
 {%- if output.output_type == 'pyout' -%}
     <div class="prompt output_prompt">
     Out[{{ cell.prompt_number }}]:
 {%- else -%}
     <div class="prompt">
 {%- endif -%}
     </div>
 {{ super() }}
 </div>
 {% endblock output %}
 
 {% block markdowncell scoped %}
+<div class="cell border-box-sizing text_cell">
 <div class="input">
 {{ self.empty_in_prompt() }}
 <div class="inner_cell">
 <div class="text_cell_render border-box-sizing rendered_html">
 {{ cell.source  | markdown2html | strip_files_prefix }}
 </div>
 </div>
 </div>
+</div>
 {%- endblock markdowncell %}
 
 {% block headingcell scoped %}
+<div class="cell border-box-sizing text_cell">
 <div class="input">
 {{ self.empty_in_prompt() }}
 <div class="inner_cell">
 <div class="text_cell_render border-box-sizing rendered_html">
 {{ ("#" * cell.level + cell.source) | replace('\n', ' ')  | markdown2html | strip_files_prefix | add_anchor }}
 </div>
 </div>
 </div>
+</div>
 {% endblock headingcell %}
 
 {% block unknowncell scoped %}
 unknown type  {{ cell.type }}
 {% endblock unknowncell %}
 
 {% block pyout -%}
 <div class="box-flex1 output_subarea output_pyout">
 {% block data_priority scoped %}
 {{ super() }}
 {% endblock %}
 </div>
 {%- endblock pyout %}
 
 {% block stream_stdout -%}
 <div class="box-flex1 output_subarea output_stream output_stdout">
 <pre>
 {{ output.text | ansi2html }}
 </pre>
 </div>
 {%- endblock stream_stdout %}
 
 {% block stream_stderr -%}
 <div class="box-flex1 output_subarea output_stream output_stderr">
 <pre>
 {{ output.text | ansi2html }}
 </pre>
 </div>
 {%- endblock stream_stderr %}
 
 {% block data_svg -%}
 {{ output.svg }}
 {%- endblock data_svg %}
 
 {% block data_html -%}
 <div class="output_html rendered_html">
 {{ output.html }}
 </div>
 {%- endblock data_html %}
 
 {% block data_png %}
 <img src="data:image/png;base64,{{ output.png }}"
 {%- if 'metadata' in output and 'width' in output.metadata.get('png', {}) %}
 width={{output.metadata['png']['width']}}
 {%- endif %}
 {%- if 'metadata' in output and 'height' in output.metadata.get('png', {}) %}
 height={{output.metadata['png']['height']}}
 {%- endif %}
 >
 {%- endblock data_png %}
 
 {% block data_jpg %}
 <img src="data:image/jpeg;base64,{{ output.jpeg }}"
 {%- if 'metadata' in output and 'width' in output.metadata.get('jpeg', {}) %}
 width={{output.metadata['jpeg']['width']}}
 {%- endif %}
 {%- if 'metadata' in output and 'height' in output.metadata.get('jpeg', {}) %}
 height={{output.metadata['jpeg']['height']}}
 {%- endif %}
 >
 {%- endblock data_jpg %}
 
 {% block data_latex %}
 {{ output.latex }}
 {%- endblock data_latex %}
 
 {% block pyerr -%}
 <div class="box-flex1 output_subarea output_pyerr">
 <pre>{{ super() }}</pre>
 </div>
 {%- endblock pyerr %}
 
 {%- block traceback_line %}
 {{ line | ansi2html }}
 {%- endblock traceback_line %}
 
 {%- block data_text %}
 <pre>
 {{ output.text | ansi2html }}
 </pre>
 {%- endblock -%}
 
 {%- block data_javascript %}
 <script type="text/javascript">
 {{ output.javascript }}
 </script>
 {%- endblock -%}
 
 {%- block display_data scoped -%}
 <div class="box-flex1 output_subarea output_display_data">
 {{ super() }}
 </div>
 {%- endblock display_data -%}