Allow Switching From Email to SMS and SMS to Email

There are a couple posts related to this, but they are both one sided (SMS to Email)... this is a feature request to bring SMS on par with the other channels we have today where we can seamlessly switch in the composer from phone to email, or messaging to email, or email to phone, etc. etc. 

SMS is disconnected from this functionality for some reason. We really need the ability to:

- Continue an SMS ticket as an email 
- Switch an email ticket to SMS

There are a variety of reasons to needing to do this, same as why we would switch from messaging to email and email to messaging, etc. We need to be able to continue the same ticket and switch the channel. Creating a new ticket/merging, etc. is just a headache and confusing for agents. 

The lack of this feature makes SMS really difficult to implement as a support channel as it doesn't behave like the other channels we have.