Template.html: remove DOCTYPE

This ensures that WebKit considers the document in 'quirks' mode which is now
needed when using document.body.scrollTop (used to check if we should scroll
down or not when adding a new message).

See https://bugs.webkit.org/show_bug.cgi?id=121419 for details.

https://bugzilla.gnome.org/show_bug.cgi?id=707781