Revert "Added download-all feature"

This reverts commit 12a8f1a672f8c6ce8f46b6267e5d836d379d9514.
Added download-all feature without instanceconfig

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 DIDN'T modified inc/instance-config.php to include the
download-all.js and jszip.min.js. That goes in the next commit.

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.