Allow for line breaks with SHIFT+RETURN

The client side of Happychat does not allow for line breaks within a single message using `SHIFT+RETURN` the way the operator interface does.

This also differs from other ares of WordPress.com (like the post/page editor) where `SHIFT+RETURN` is functional, which is a bit of a disconnect for some users who are used to this option on our platform.

Reported in 1988204-hc

Start by locating the Happychat client’s message input and keyboard-event handling, then compare it with the operator interface’s line-break behavior. Done means SHIFT+RETURN inserts a line break within one message without sending it, matching the behavior described in the issue.

Written by the indexing model from the issue text.