Context

we would like to be able to tell admins about new features they might want to use
we also want to respect people's ability to consent to receiving communications from us
so: when people create a channel, invite them to subscribe to the support channel IFF a support channel exists on the given signalboost instance AND they are not already subscribed to it

Changes

add registrar.channel._inviteToSupportChannel:
- in registrar.channel.create, check to see if there is a support channel, if so..
- invite all new channel admins to subscribe to the support channel (unless they are already members)
GOTCHA protection: strip extraneous strings in suport chan env var
- it is super easy to assume that the SIGNALBOOST_SUPPORT_NUMBER env var string should be surrounded by quotes (because strings usually are!)
- however, if a dev provides quotes around the number, the quote characters will be interpreted literally, causing the support channel phone number to have 2 extra characters and not be a valid e164 phone number
- to avoid this, strip quotation marks from the env var variable when using it to assign to supportPhoneNumber in app.config.signal
- additionally: strip quote marks from example support channel env var in .env.dev.gpg to avoid misleading future devs!