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/main/views/manage_users.py

@@ -65,6 +65,11 @@ def invite_user(service_id, user_id=None):
 
     if user_id:
         user_to_invite = User.from_id(user_id)
+        if user_to_invite.belongs_to_service(current_service.id):
+            return render_template(
+                'views/user-already-invited.html',
+                user_to_invite=user_to_invite,
+            )
         form.email_address.data = user_to_invite.email_address
     else:
         user_to_invite = None

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 %}