Implement 3-way compose box resizing

[alya]

As part of the compose box redesign, we should implement a 3-way compose box resizing button:

• The first click expands the compose box half-way (new state).
• The second click expands it all the way.
• The third click collapses the compose box to its default (open) size.

There are different icons for each of these states. See live demo.

Notes:

• We may need to experiment with how tall the intermediate-size compose box is; see demo for a starting point.
• As a follow-up, we should be able to remove the ability to drag the corner to stretch the compose box. This is likely to be helpful for moving the compose box buttons into the text area and having that look good.

Blockers:

• Move compose expand/collapse button into text area #28791

This issue is part of #24878.

CZO discussion

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Live demo with the source code on Repl.it. Here is the decomposed source code archive:
compose-decomposed.zip

The css code is split to decomposed areas but all the js code is in a single file. The file isn't big so it is easier to follow the logic of interactions between html and js.

[zulipbot]

Hello @zulip/design members, this issue was labeled with the "redesign" label, so you may want to check it out!