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.