Filed #187, #188, and #190.

Other items already existed as #80, #82, #95, #122, and #123,
or were already complete (a compose box, and three ways of
attaching something to a message.)

This means that the overall scroll view has AxisDirection.down
as the scroll direction, rather than AxisDirection.up, and the
SliverStickyHeader is passed GrowthDirection.reverse instead
of GrowthDirection.forward in order to get the same effect.

This makes the scrolling a little easier to think about when focused
on MessageList's code, because for example (as seen in this diff)
`scrollMetrics.extentBefore` now refers to the messages that are
older than the ones on screen, rather than those that are newer.

This also brings us closer to the setup we'll want for zulip#80 and zulip#82,
opening the message list at an anchor other than the end: the empty
placeholder sliver at the bottom will become the list of messages
after the starting anchor, while the sliver at the top will remain
the list of messages above the anchor.

This means that the overall scroll view has AxisDirection.down
as the scroll direction, rather than AxisDirection.up, and the
SliverStickyHeader is passed GrowthDirection.reverse instead
of GrowthDirection.forward in order to get the same effect.

This makes the scrolling a little easier to think about when focused
on MessageList's code, because for example (as seen in this diff)
`scrollMetrics.extentBefore` now refers to the messages that are
older than the ones on screen, rather than those that are newer.

This also brings us closer to the setup we'll want for zulip#80 and zulip#82,
opening the message list at an anchor other than the end: the empty
placeholder sliver at the bottom will become the list of messages
after the starting anchor, while the sliver at the top will remain
the list of messages above the anchor.

This means that the overall scroll view has AxisDirection.down
as the scroll direction, rather than AxisDirection.up, and the
SliverStickyHeader is passed GrowthDirection.reverse instead
of GrowthDirection.forward in order to get the same effect.

This makes the scrolling a little easier to think about when focused
on MessageList's code, because for example (as seen in this diff)
`scrollMetrics.extentBefore` now refers to the messages that are
older than the ones on screen, rather than those that are newer.

This also brings us closer to the setup we'll want for #80 and #82,
opening the message list at an anchor other than the end: the empty
placeholder sliver at the bottom will become the list of messages
after the starting anchor, while the sliver at the top will remain
the list of messages above the anchor.