Prevent button click when enter key is pressed in LDAP wizard

Pressing enter in the LDAP wizard will trigger a click on the first
button. In the main page it would trigger the delete dialog, which is
quite inconvenient.

Added a type attribute to suppress this behavior.

Fixes https://github.com/owncloud/core/issues/11439

Please review @blizzz @MorrisJobke @Xenopathic