Make our class attribute values more consistent

[jeremyboggs]

Watching/listening to @shane-et-al talk about web scraping in CodeLab, I notice that we have a rather inconsistent way of encoding class attribute values. Take any given blog post, for example, and we have the following:

Value with a single dash between post and the label for a particular thing:
<h1 class="post-title">...</h1>

Value with only the label for a particular thing:
<span class="author">...</span>

Value that uses a single underscore between post and the label for a particular thing:
<div class="post_crosspost">...</div>

Value that uses two underscores between post and the label for a particular thing:
<footer class="post__credits">...</footer>

We should make this more consistent. :D