add possibility to turn off new registrations (#2017)

with LNBITS_ALLOW_NEW_ACCOUNTS=false

while keeping existing users functional (i.e. no allowlist for existing users)

lnbits/settings.py

@@ -35,6 +35,11 @@ class LNbitsSettings(BaseModel):
 class UsersSettings(LNbitsSettings):
     lnbits_admin_users: List[str] = Field(default=[])
     lnbits_allowed_users: List[str] = Field(default=[])
+    lnbits_allow_new_accounts: bool = Field(default=True)
+
+    @property
+    def new_accounts_allowed(self) -> bool:
+        return self.lnbits_allow_new_accounts and len(self.lnbits_allowed_users) == 0
 
 
 class ExtensionsSettings(LNbitsSettings):