Add support for spoilers

[timabbott]

See https://github.com/zulip/zulip/pull/15281; we recently added markdown support for the spoilers feature, and it'd be valuable for zulip-terminal to at least not display the hidden text by default.

[neiljp]

@timabbott Thanks for filing this, I appreciate the heads-up.

At absolute minimum for v0 I'd expect us to hide the spoiler content, perhaps providing a link to the message on the server, though that's not necessary?

I'd suggest one v1 approach might be to integrate this with implementing a fix for #543 (for 'large' messages), showing the 'full' text in that view. That does combine two use-cases (see all of a message & see spoilers), which we could avoid by having a 'cycle' interface (not just toggle, but one key cycles, or exit with esc; or the new idea I raised for help/about of left/right panes/tabs/popups)

Another v1 approach may be to put this in the scope of viewing 'extended content', ie. listing images, links, etc. However, that loses the context in the message.

[neiljp]

This was discussed in #zulip-terminal>Spoilers! #T688 #T1061 #T1173 on chat.zulip.org.