Remove assumption that the table of contents will be an <ol>.

A recent change to anolis[1] caused it to output the table of contents
using `<ul>` tags instead of `<ol>` tags, breaking the generation of the
table of contents for individual pages.

Instead, we now search for the table of contents by the toc class
without assuming what tag it will use.

[1] https://bitbucket.org/ms2ger/anolis/commits/62d9a108439d70e33ad2bb3b4009efd3dbf9cac5