Diffs in Images and Whitespace

* Make image tags a 'word'
* Make whitespace characters words

The former allows an image to change without mangling it on the front end. The latter allows us to use different types of whitespace characters; before words would include newline characters, which caused strange edits.

Also fixed some tests which had off-by-one errors. These errors cascading into the front-end (see https://github.com/eregs/regulations-site/pull/383)