Add more compose buttons: code, spoiler, math, bold, etc. etc.

[gnprice]

Zulip web now has a long list of buttons at the bottom of the compose box to help make it easy to insert markup for Zulip's many message-content features:

We should add more compose buttons too; not necessarily the exact same set as web, but a lot more than we currently have: just three, all for uploading files to attach to the message.

These buttons already appear in the design in Figma (cf #915):

https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3954-13395&m=dev

so this is mainly an umbrella issue for adding the various buttons.

The first prerequisite step will be to work out an idea of just what set of buttons we want. Then we can file separate issues for the individual buttons.

There's also one aspect of the design which we left out in #915, and will become necessary when we have a few more buttons:

• the "..." overflow button, scrolling, and fading for excess compose buttons — for now we have just three of them, not enough to make those come into effect

This is an issue we won't get to before the app is launched, because the legacy app doesn't have those other buttons either.

[gnprice]

We had a user request for these the other day:
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1961857
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1963791

In particular they point out that backticks are often annoying to type on a mobile keyboard.