Allow "hide terms" in Classify interface

When classifying a new article (e.g. http://slim.icr.ac.uk/articles/classify/?pmid=8610146),,) the "show terms" button on "Article classification" allows highlighting of the MeSH terms on the abstract. However, once they are shown, it is not possible to hide them.

Either make the "Show terms" into an on/off slider or change the button to "Hide terms" when appropriate.