Make standard scrollable popup hotkeys more obvious

[neiljp]

Currently we indicate in the help menu that ? 'show/hide the help menu', and indicate that 'up/down scrolls' in the title - and similarly for many other standard popups.

This has a few issues:

• the popup title is slightly mixed with the scrolling hint
• except for the help menu, there is no hint how to exit a popup

Current proposed solution: Add a section in the footer of popups which shows something like

• Scroll [up][down] (or some variation suggesting all scrolling keys work?)
• Exit popup [<POPUP_KEY>, Esc] (specific to each popup)
• Return to <PARENT> [<PARENT_KEY>] (only if popup used as a child, eg. f from message info)

This would enable removing the scrolling text from each popup title, making it clearer and more central.

Note that each popup would require a minimum width for this, at least for the first and second points. The sometimes-present parent entry could be on a row below the others, or centered between them, which can be finalized after exploring if there is space for the latter or whether it looks too cluttered when present.

The proposed solution above may need to be adjusted slightly for child popups, since the design may not precisely match the current behavior.

[zulipbot]

Hello @zulip/server-hotkeys members, this issue was labeled with the "area: hotkeys" label, so you may want to check it out!