Short circuit if already a team member

It would be confusing if people got invited twice, so let’s tell people
if someone’s already been invited.

app/templates/views/user-already-invited.html

@@ -0,0 +1,19 @@
+{% extends "withnav_template.html" %}
+{% from "components/page-header.html" import page_header %}
+
+{% block service_page_title %}
+  This person is already a team member
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  {{ page_header(
+    '{} is already a team member'.format(user_to_invite.name),
+    back_link=url_for('main.manage_users', service_id=current_service.id)
+  ) }}
+
+  <p class="govuk-body">
+    Someone’s already invited them. You do not need to do anything.
+  </p>
+
+{% endblock %}