[PM-24273] Milestone 2C (#6544)

* feat(billing): add mjml template and updated templates

* feat(billing): update maileservices

* feat(billing): add milestone2 discount

* feat(billing): add milestone 2 updates and stripe constants

* tests(billing): add handler tests

* fix(billing): update mailer view and templates

* fix(billing): revert mailservice changes

* fix(billing): swap mailer service in handler

* test(billing): update handler tests

src/Core/MailTemplates/Mjml/emails/invoice-upcoming.mjml

@@ -0,0 +1,27 @@
+<mjml>
+  <mj-head>
+    <mj-include path="../components/head.mjml" />
+  </mj-head>
+
+  <mj-body background-color="#f6f6f6">
+    <mj-include path="../components/logo.mjml" />
+
+    <mj-wrapper
+      background-color="#fff"
+      border="1px solid #e9e9e9"
+      css-class="border-fix"
+      padding="0"
+    >
+      <mj-section>
+        <mj-column>
+          <mj-text>
+ Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc semper sapien non sem tincidunt pretium ut vitae tortor. Mauris mattis id arcu in dictum. Vivamus tempor maximus elit id convallis. Pellentesque ligula nisl, bibendum eu maximus sit amet, rutrum efficitur tortor. Cras non dignissim leo, eget gravida odio. Nullam tincidunt porta fermentum. Fusce sit amet sagittis nunc.
+
+          </mj-text>
+        </mj-column>
+      </mj-section>
+    </mj-wrapper>
+
+    <mj-include path="../components/footer.mjml" />
+  </mj-body>
+</mjml>