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/models.py

@@ -410,7 +410,7 @@ class User(Base, Email):
         return emails
 
     def send_welcome(self):
-        if app.config["WELCOME"].lower() == "true":
+        if app.config["WELCOME"]:
             self.sendmail(app.config["WELCOME_SUBJECT"],
                 app.config["WELCOME_BODY"])