Activate submit_btn.

def _contents_changed(self):
        """Activate submit_btn."""
        desc_chars = (len(self.input_description.toPlainText()) -
                      self.initial_chars)
        if desc_chars < DESC_MIN_CHARS:
            self.desc_chars_label.setText(
                u"{} {}".format(DESC_MIN_CHARS - desc_chars,
                                _("more characters to go...")))
        else:
            self.desc_chars_label.setText(_("Description complete; thanks!"))

        title_chars = len(self.title.text())
        if title_chars < TITLE_MIN_CHARS:
            self.title_chars_label.setText(
                u"{} {}".format(TITLE_MIN_CHARS - title_chars,
                                _("more characters to go...")))
        else:
            self.title_chars_label.setText(_("Title complete; thanks!"))

        submission_enabled = (desc_chars >= DESC_MIN_CHARS and
                              title_chars >= TITLE_MIN_CHARS)
        self.submit_btn.setEnabled(submission_enabled)