Fully resume autocomplete if choosing to resume compose from confirmation popup

[neiljp]

Bug description

Since #1442, we now prompt with a popup under certain circumstances when a user uses the key to exit compose, and allow resuming compose depending upon the result.

However, while work in that PR did originally support maintaining the footer content if returning to compose from the popup, it needed further support than that to maintain the autocomplete state and resume it upon returning to the compose box, so this aspect was dropped from the PR.

The current status results in the autocomplete text at the time of the popup being triggered being left in the compose box - this does not lose information, and should only be an inconvenience, since it also leaves the cursor position at the previous location.

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See #1442 for discussion, as well as the Improve handling Esc during msg compose #T1342 #T1362 #T1442 topic in #zulip-terminal.