Stop automatically resending email verification links

This commit stops a new email verification link from being sent to a
user if they click on an email link which has expired or which has
already been used. Instead, they will be see an error message with a
link to the sign in page. This stops the situation where someone could
log in indefinitely (without the needing to enter their password) by
trying to use a used / expired email verification link and receiving a
valid link automatically.

app/templates/views/email-link-invalid.html

@@ -0,0 +1,19 @@
+{% extends "withoutnav_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block per_page_title %}
+  Invalid email link
+{% endblock %}
+
+{% block maincolumn_content %}
+
+<div class="grid-row">
+  <div class="column-two-thirds">
+    <h1 class="heading-large">The link has expired</h1>
+
+    <p><a href="{{ url_for('main.sign_in') }}">Sign in again</a> to get a new link.</p>
+
+  </div>
+</div>
+
+{% endblock %}