Agent Workspace: Add "last channel used" as default reply option

Feature Request Summary: 

It would be great to have the option to set the channel to default to "last channel used" within ticket settings, so that if an end user replies to a messaging conversation by email, an agent automatically has email as the default public reply channel.

Description/Use Cases: 

Agent workspace allows for easy communication with customers on multiple channels, and to easily change between channels based on a customer's preferences.

Right now, the default reply channel is "messaging" if messaging has been used in a conversation. In our business, many of these change over to "email" as the continued form of communication if further follow-up is needed.

At present, it is necessary to change the channel from "messaging" to "email" every time an email is sent, and the last channel used is not persistent each time a ticket is handled by an agent.

Business impact of limitation or missing feature:

Around 30% of our messaging conversations change to email after the initial conversation is started. Having the default be messaging every time a ticket is returned to makes it so that sometimes a message is accidentally sent when the intended channel of reply is email. (Messages are sent by hitting enter, so if the channel hasn't been changed prior to typing an email, it's easy to send a message that simply is the beginning of an email.)

We utilize continuous conversations so the customer does receive a copy of the message sent via messaging, but the end user experience is different between notification of receiving an email update to a ticket vs. notification of receiving a new message in a conversation. When email updates are sent, the prior history of the conversation is present, while with the continuous conversation notification, the prior history is not present.  (We have excluded "update via messaging" from our email notifications to prevent duplicate emails to end users.)

The changing back and forth between channels rather than switching from messaging to email seamlessly can be confusing for the end user and we'd like to avoid it by changing over to email as soon as the end user changes to email.

Having the default set based on last channel used would also improve agency efficiency by reducing overall handling time of a ticket.

Adding this to the ticket settings options in the form of "default public reply channel is last channel used" that was recessed under the option to have the default reply as public reply, would improve both the agent and the end user experience.