Bugfix: wp was added after using the view buttons

A bug was introduced in commit 8abedcd6cc6a217a62a903ed7dc0f7a0b696caab to the
"Add new word pair" page, because it changed the view buttons ("Simple
view"/"Advanced view") from "span" to "button". Since (at least in Chromium)
the default button type is "submit", the "add word pair" HTML form was
submitted when these buttons were pressed, which caused the word pair to be
added.