Prevent email enumeration (#1000)

Here is my attempt to prevent user enumeration. 
I've made the following changes:

**Application:**
- replaces success and failure messages in the form with two (with/without redis) information messages 
- adds logging for attempted password resets of unknown users
- adds logging for failing emails

**Tests:**
- test_unknown asserts a redirect instead of a ok
- adds test_email_reset_twice_redis to assert the correct logging of a twice reset email 
- adds a FakeRedis class similiar to the one implemented in test_metrics.py. I could refactor them into the testutils folder if prefered. 

Please excuse the commit mess. I am currently fighting with my tooling.

src/pretix/base/forms/auth.py

@@ -180,12 +180,4 @@ class PasswordForgotForm(forms.Form):
         super().__init__(*args, **kwargs)
 
     def clean_email(self):
-        email = self.cleaned_data['email']
-        try:
-            self.cleaned_data['user'] = User.objects.get(email=email)
-            return email
-        except User.DoesNotExist:
-            raise forms.ValidationError(
-                _("We are unable to find a user matching the data you provided."),
-                code='unknown_user'
-            )
+        return self.cleaned_data['email']