extend POST /users/:id

Now you can set a password for a user while the server is running,
e.g. via the following command:

```shell
curl \
    -H 'Content-Type: application/json' \
    -d '{"update": {"password": "asdf"}}' \
    --unix-socket /var/lib/moonfire-nvr/sock \
    http://nvr/api/users/1
```

server/db/proto/schema.proto

@@ -72,4 +72,8 @@ message Permissions {
   bool read_camera_configs = 2;
 
   bool update_signals = 3;
+
+  // Administrate user accounts: create, delete accounts; modify passwords of
+  // accounts other than the caller's own.
+  bool admin_users = 4;
 }