Rename billing to payment throughout the codebase (#726)

* Rename billing to payment throughout the codebase

This change updates terminology from "billing" to "payment" to better
reflect that these are contributions/payments rather than bills.

Changes include:
- Rename BillingsController to PaymentsController
- Rename billing_email to payment_email
- Rename next_billing_date to next_payment_date
- Rename create_billing_portal_session_url to create_payment_portal_session_url
- Update routes from billing to payment
- Update all 12 locale files with new terminology
- Update views, helpers, and tests

* Update app/views/subscriptions/upgrade.html.erb

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Juan José Mata <jjmata@jjmata.com>

---------

Signed-off-by: Juan José Mata <jjmata@jjmata.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/views/subscriptions/upgrade.html.erb

@@ -52,7 +52,7 @@
         ) %>
 
         <p class="text-xs text-secondary">
-          In the next step, you'll be redirected to Stripe which handles our billing.
+          In the next step, you'll be redirected to Stripe which handles our payments.
         </p>
       </div>
     <% end %>