Create a single conversion script for generating html

The original project had a series of shell and python scripts to generate html files from the notebooks. Some of that was really hacky, because I didn't know how Jupyter/ IPython notebooks were generated in enough detail. In this issue, I'll pull this into one Python script, that uses nbconvert and templates as much as possible. All remaining conversion work will be done in Python.

[x] Create base Python conversion script.
[x] Base conversion script in root project folder. /notebooks folder should only contain notebooks. Other resources should be in folders under project root.

Incorporate the functionality of all the individual .sh and .py files in scripts/ into build_html_site.py, and templates. When all of this functionality has been updated and simplified, these scripts can be deleted.

[x] build_html_pages.sh
- calls all of the other scripts, this should be checked off last
[x] sed section of build_html_pages.sh to make output display initially on Mapping Global Earthquake Activity
- Add a tag that allows specific code cells to show output by default?
- Add a tag that allows specific notebooks to show all output initially by default?
[x] (keeping this) build_all_exercises_page.py
- May just keep this as a separate issue for completion.
[x] create_common_hmtl.sh
- The default nbconvert behavior is to link to other notebooks on nbviewer. This script modifies the html output to link to other html notebooks. (Make sure links between notebooks work.)
[x] highlight_code.py
- highlight specified lines of code
[x] highlight_inline_code.sh
- highlight code snippets in bodies of text
[x] show_hide_output.py
- This is being taken care of in the template now. But the script only included show/hide all buttons if there was an output div at some point in the notebook. That might be good to implement. (see #148)
[x] convert_home_links.py
- This script converts links from "index.html" to "/". The server responds by serving the index page. Difference between requesting introtopython.org/index.html and introtopython.org.
[x] remove_input_references.py
[x] build_index.py
- look at actual code before checking this off

ehmatthes / intro_programming

Create a single conversion script for generating html #138