msglist: Scroll-to-bottom button on message list

[sirpengi]

The message list should offer a clickable button that allows the user to scroll to the bottom (to the latest message). This button should hide itself when the user is at the bottom of the message list.

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Notes

The icon used on web+mobile appear to be fa-chevron-down but that isn't available in the Materials icons (maybe linked to #200 )

The web version has additional logic that hides the button for keyboard users (see https://github.com/zulip/zulip/blob/1cd587d24be1d668fcf6d136172bfec69e35cb75/web/src/message_scroll.js) but I don't believe this is necessary/desired in the mobile app context

[gnprice]

Thanks!

The icon used on web+mobile appear to be fa-chevron-down but that isn't available in the Materials icons (maybe linked to #200 )

Yeah. In that case just pick the closest approximation you can find in the Material icon set, and leave a TODO pointing at #200.

The web version has additional logic that hides the button for keyboard users […] but I don't believe this is necessary/desired in the mobile app context

Yeah, I agree.

[sirpengi]

This was implemented in #223
Thanks for everybody that helped out!