Added download-all feature

The button to download all the files in a thread only appears when the
page is a thread, such as treeview. I added the download-all button next
to the treeview one. I modified stylesheets/v8ch.css to include the id
#download-all. I modified inc/instance-config.php to include the
download-all.js and jszip.min.js. I added the files js/jszip.min.js
which is a library to work with zip files in JS and I added
js/download-all.js which goes through all files, adds them to a zip and
then gives it to the user.