Add human delay option for sending messages

[ericnewcomer]

We've discussed this at times in the past but wanted to have an issue to track and discuss this feature oft requested customers. The gist here is they want to appear more human by not replying back faster than somebody can type.

Other considerations would include typing notification on channels that support it.

Last consensus was around adding a channel level delay option.
See: https://github.com/rapidpro/rapidpro/issues/875#issuecomment-882667296