msglist: Spoilers should always grow downward when they open

[chrisbobbe]

With #503, we'll have support for spoilers! It has a glitch that won't be straightforward to fix, though, so I'm filing this to be worked on later.

The message list's growth direction is up, not down. As a result, when you open a spoiler, it grows up, but it would be better if it grew down, in the direction of the "expand" arrow. Here's what #503 looks like:

It's possible that we'll eventually build the message list out of two back-to-back slivers, with opposite growth directions, instead of just one sliver that grows upward. This approach is being considered for a few issues:

• Open message list at first unread, rather than latest message #80
• Open a message list in the middle of history, when requested #82
• Control scroll position on new and newly-fetched messages #83

If that happens, then I guess spoilers might sometimes naturally grow downward, but only if they happened to be in the sliver that grows downward.

[gnprice]

Reported in chat: https://chat.zulip.org#narrow/stream/48-mobile/topic/flutter.20spoilers.20pin.20scroll.20at.20bottom/near/1799494