compose: Hide upload-file buttons when `max_file_upload_size_mib` is zero

[chrisbobbe]

It's not yet in the API doc (CZO discussion: #api documentation > max_file_upload_size_mib being zero @ 💬), but max_file_upload_size_mib in the /register response can be zero, and when it is, clients should exclude the upload buttons in the compose box.

This issue is for doing that.

We should also update the relevant Help Center doc and the production doc.

[vivekTiw120303]

Hello, is this available to work, i wanted to contribute here

[aikansh008]

hii, i want to contribute to this bug i am also an open source contributor at GSSOC

[chrisbobbe]

Please take a look at the Zulip project's guide to getting involved with the code:

https://zulip.readthedocs.io/en/latest/contributing/contributing.html#your-first-codebase-contribution
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on

[vivekTiw120303]

So this issue has been assigned or still open to work?