Merge #2328

2328: Feature: Configurable default spam threshold used for new users r=mergify[bot] a=enginefeeder101

## What type of PR?

Feature

## What does this PR do?

This PR adds functionality to set a custom default spam threshold
for new users. The environment variable ``DEFAULT_SPAM_THRESHOLD`` is
used for this purpose. When not set, it defaults back to 80%, as the
default value was before.

If ``DEFAULT_SPAM_THRESHOLD`` is set to a value that Python cannot
parse as an integer, a ValueError is thrown. There is no error handling
for that case built-in. Should that be done?

## Prerequisites
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/workflow.html#changelog) entry file.


Co-authored-by: enginefeeder101 <enginefeeder101@users.noreply.github.com>
Co-authored-by: Dimitri Huisman <diman@huisman.xyz>

core/admin/mailu/configuration.py

@@ -76,6 +76,7 @@ DEFAULT_CONFIG = {
     'CREDENTIAL_ROUNDS': 12,
     'TLS_PERMISSIVE': True,
     'TZ': 'Etc/UTC',
+    'DEFAULT_SPAM_THRESHOLD': 80,
     # Host settings
     'HOST_IMAP': 'imap',
     'HOST_LMTP': 'imap:2525',
@@ -163,6 +164,7 @@ class ConfigManager:
         self.config['MESSAGE_RATELIMIT_EXEMPTION'] = set([s for s in self.config['MESSAGE_RATELIMIT_EXEMPTION'].lower().replace(' ', '').split(',') if s])
         self.config['HOSTNAMES'] = ','.join(hostnames)
         self.config['HOSTNAME'] = hostnames[0]
+        self.config['DEFAULT_SPAM_THRESHOLD'] = int(self.config['DEFAULT_SPAM_THRESHOLD'])
 
         # update the app config
         app.config.update(self.config)