Disallow cycling from recipient box using arrow keys if recipient is invalid

[kaustubh-nair]

Follow-up from #738, we should also disable switching from recipient box using arrow keys, in addition to currently supported CYCLE_COMPOSE_BOX key, if the recipient is invalid.

[zee-bit]

@kaustubh-nair @neiljp I have a doubt/confusion. The arrow keys do not just cycle the focus between corresponding compose message blocks(like Tab) but are also used to move the cursor inside a particular block(to edit contents). So how would we figure out if an arrow keypress is meant to cycle the focus between message compose blocks or just to move the cursor inside a specific block?

[neiljp]

I believe the text box should track the cursor location, which should enable checking whether it's at the maximum left/right/top/bottom of the box.

[zee-bit]

Oh. I didn't know we could determine the cursor position inside the box using header.edit_pos. I would like to work on this issue then. @zulipbot claim