Config variable for silent/non-silent induction outside of "/induct" command

After #54, there are now 2 other ways for user's to be inducted outside of the `/induct` command: a user self-inducting via the `/make-member` command and a committee member using `/ensure-members-inducted`. The code to send the welcome message upon user induction is defined within the `/induct` command call-back, therefore it is not called if the user is inducted any other way. This essentially acts like a silent induction.
This code should be extracted in order for it to be called in any induction method.
In addition an environment variable should allow the configuration of whether the alternative induction methods act like a silent or non-silent induction.

Comienza leyendo los callbacks de /induct, /make-member y /ensure-members-inducted, centrándote en dónde se envía actualmente el mensaje de bienvenida de induction. Extrae el comportamiento compartido y añade la configuración solicitada mediante una variable de entorno para que las rutas de induction alternativas puedan ser silenciosas o no silenciosas; se considera terminado cuando las tres rutas siguen el comportamiento configurado.

Escrito por el modelo de indexación a partir del texto del issue.