Unify and coerce booleans from env used in admin

At some places, the string that DOMAIN_REGISTRATION is got used like a boolean
(an easy misassumption to make while in python and dealing with the config
dict), making `DOMAIN_REGISTRATION=False` act as a truthy value. To stop such
future problems from happening, coerce environment config strings to real
bools.

closes #830

core/admin/mailu/configuration.py

@@ -30,11 +30,11 @@ DEFAULT_CONFIG = {
     'POSTMASTER': 'postmaster',
     'TLS_FLAVOR': 'cert',
     'AUTH_RATELIMIT': '10/minute;1000/hour',
-    'DISABLE_STATISTICS': 'False',
+    'DISABLE_STATISTICS': False,
     # Mail settings
     'DMARC_RUA': None,
     'DMARC_RUF': None,
-    'WELCOME': 'False',
+    'WELCOME': False,
     'WELCOME_SUBJECT': 'Dummy welcome topic',
     'WELCOME_BODY': 'Dummy welcome body',
     'DKIM_SELECTOR': 'dkim',
@@ -74,13 +74,21 @@ class ConfigManager(dict):
     def __init__(self):
         self.config = dict()
 
+    def __coerce_value(self, value):
+        if isinstance(value, str) and value.lower() in ('true','yes'):
+            return True
+        elif isinstance(value, str) and value.lower() in ('false', 'no'):
+            return False
+        return value
+
     def init_app(self, app):
         self.config.update(app.config)
         # get environment variables
         self.config.update({
-            key: os.environ.get(key, value)
+            key: self.__coerce_value(os.environ.get(key, value))
             for key, value in DEFAULT_CONFIG.items()
         })
+
         # automatically set the sqlalchemy string
         if self.config['DB_FLAVOR']:
             template = self.DB_TEMPLATES[self.config['DB_FLAVOR']]