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.

/induct、/make-member、/ensure-members-inducted の callback を読み、現在 induction welcome message がどこで送信されているかに注目してください。共通する動作を抽出し、代替の induction path を silent または non-silent にできるよう、要求された環境変数の設定を追加してください。3 つすべての path が設定された動作に従えば完了です。

索引モデルが issue の本文から書いたものです。