Reformat all paragraphs to be 80 characters wide

Most of the formatting here was already readable, but a number of paragraphs broke at random points ([example](https://github.com/mozilla/webdev-bootcamp/pull/33/files#r6957100)). I went a little overboard and just reformatted everything.

These changes should not affect the generated output. I used `make singlehtml`, screenshots, and [ImageMagick compare](http://www.imagemagick.org/Usage/compare/) to compare the generated content (note *content* -- this approach does not take things like link destinations into account) and confirmed that the only change was a line number citation at the end of the *Todo* section, which changed because my reformatting shortened `coding.rst`.