[bugfix] dont send welcome message as support channel invite

• SYMPTOM: new admins receive a welcome message from the support channel. this is confusing b/c it implies that they are admins of the support channel and actively bad b/c it prevents them from ever getting an invite notice from the support channle
• CAUSE: we accidentally swapped _welcomeNotificationOf for inviteReceived when doing an unrelated refactor of `sdMessageOf`` (oh noes!)
• FIX: restore invite string and make it more explicit (since we no longer can lean on the channel name to provide context)

EXTRA CREDIT: localize welcome message and support channel invite, clean up specs