Merge #1754

1754: centralize Webmail authentication behind the admin panel (SSO) r=mergify[bot] a=nextgens

## What type of PR?

Enhancement: it centralizes the authentication of webmails to the admin interface.

## What does this PR do?

It implements the glue required for webmails to do SSO using the admin interface.
One of the main advantages of centralizing things this way is that it reduces significantly the attack surface available to an unauthenticated attacker (no webmail access until there is a valid Flask session).

Others include the ability to implement 2FA down the line and rate-limit things as required.

### Related issue(s)
- #783

## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

core/admin/mailu/models.py

@@ -11,6 +11,7 @@ import sqlalchemy
 import time
 import os
 import glob
+import hmac
 import smtplib
 import idna
 import dns
@@ -458,6 +459,15 @@ in clear-text regardless of the presence of the cache.
         user = cls.query.get(email)
         return user if (user and user.enabled and user.check_password(password)) else None
 
+    @classmethod
+    def get_temp_token(cls, email):
+        user = cls.query.get(email)
+        return hmac.new(app.temp_token_key, bytearray("{}|{}".format(datetime.utcnow().strftime("%Y%m%d"), email), 'utf-8'), 'sha256').hexdigest() if (user and user.enabled) else None
+
+    def verify_temp_token(self, token):
+        return hmac.compare_digest(self.get_temp_token(self.email), token)
+
+
 
 class Alias(Base, Email):
     """ An alias is an email address that redirects to some destination.