upgrade: Rename "Add card" button to "Pay now".

We do not redirect to Zulip's billing page for confirmation
even when user did not had added a card already and the payment
is directly processed after adding a card so is better to just
rename the button.

templates/corporate/upgrade.html

@@ -158,7 +158,7 @@
                                 <div class="upgrade-button-container" {% if is_demo_organization %}data-tippy-content="{% trans %}Convert demo organization before upgrading.{% endtrans %}"{% endif %}>
                                     <button id="add-card-button" class="stripe-button-el invoice-button" {% if is_demo_organization %}disabled{% endif %}>
                                         <span id="add-card-button-span">
-                                            Add card
+                                            Pay now
                                         </span>
                                     </button>
                                 </div>