Add interstitial page before using email auth token

Some email clients will pre-fetch links in emails to check whether
they’re safe. This has the unfortunate side effect of claiming the token
that’s in the link.

Long term, we don’t want to let the link be used multiple times, because
this reduces how secure it is (eg someone with access to your browser
history could re-use the link even if you’d signed out).

Instead, this commit adds an extra page which is served when the user
clicks the link from the email. This page includes a form which submits
to the actual URL that uses the token, thereby not claiming the token as
soon as the page is loaded.

For convenience, this page also includes some Javascript which clicks
the link on the user’s behalf. If the user has Javascript turned off
they will see the link and can click it themselves. This is going on the
assumption that whatever the email clients are doing when prefetching
the link doesn’t involve running any Javascript.

This Javascript is inlined so that:
- it is run as fast as possible
- it’s more resilient – even if our assets domain is unreachable or the
  connection is interrupted, it will still run

app/main/views/two_factor.py

@@ -28,7 +28,12 @@ def two_factor_email_sent():
     )
 
 
-@main.route('/email-auth/<token>', methods=['GET', 'POST'])
+@main.route('/email-auth/<token>', methods=['GET'])
+def two_factor_email_interstitial(token):
+    return render_template('views/email-link-interstitial.html')
+
+
+@main.route('/email-auth/<token>', methods=['POST'])
 def two_factor_email(token):
     if current_user.is_authenticated:
         return redirect_when_logged_in(platform_admin=current_user.platform_admin)