Add "magic link" functionality

When using a device that is bothersome to log in on (e.g. a Kindle) you can use a magic link to log in via another device.

Configuration was added and is disabled by default.

cps/redirect.py

@@ -0,0 +1,25 @@
+# http://flask.pocoo.org/snippets/62/
+
+from urlparse import urlparse, urljoin
+from flask import request, url_for, redirect
+
+
+def is_safe_url(target):
+    ref_url = urlparse(request.host_url)
+    test_url = urlparse(urljoin(request.host_url, target))
+    return test_url.scheme in ('http', 'https') and ref_url.netloc == test_url.netloc
+
+
+def get_redirect_target():
+    for target in request.values.get('next'), request.referrer:
+        if not target:
+            continue
+        if is_safe_url(target):
+            return target
+
+
+def redirect_back(endpoint, **values):
+    target = request.form['next']
+    if not target or not is_safe_url(target):
+        target = url_for(endpoint, **values)
+    return redirect(target)