ComposeMenu [nfc]: Convert to functional component.

[WesleyAC]

This was step one in a UX change I was planning to make. I got sidetracked due to problems with the library I was planning to use (callstack/react-native-paper#2769), but I figure I'd push this refactor commit anyways.

[chrisbobbe]

Thanks! See one small comment below.

[chrisbobbe]

nit: This useCallback isn't doing much; uploadFileCallback's callers are functions that get recreated each time the ComposeMenu renders (they aren't using useCallback), and those functions are being passed to various children.

[gnprice]

Thanks for this refactor! I agree with Chris's small comment, and everything else looks good.

Other than cutting that useCallback which isn't doing anything useful, the other direction to go would be to apply useCallback to all the named functions there, so that the callbacks passed as props to child components would be made stable. That's potentially an optimization. But we had a chat discussion a couple of weeks ago about when to use useCallback:
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/React.2EuseCallback/near/1226466
and one conclusion was that that optimization may often not be effective at all. If it is effective here, it's unlikely to be meaningful -- it doesn't seem like these icon components (which are what ultimately get some of these callbacks as props) should have much work at all to do in their render functions.