Allow using signatures for all channels

[gavry]

While this won't be useful for everyone, I think it's a nice preference to allow more formality with multiple channels.

Use cases:

• Our API channel (contact form) responds to customers via Email; this would be useful to standardize
• Our web chat uses email continuity. Would be great to have a signature

Even better (but more advanced), would be to have multiple saved signatures, and the ability to choose within the message box. So then you could have a shorter one for SMS ( - Joe, Starbucks ), and so on.

Thanks for your consideration!