Warning message before unsubscribing from streams

[arijitghosal03]

An alert has been added when user wises to unsubscribe from a stream.
Changes made in StreamScreenSettings.js

[gnprice]

Thanks for the contribution.

It sounds like this is intended for #5553. Do mention the issue in the PR and commit; see style guide for details:
https://github.com/zulip/zulip-mobile/blob/main/docs/style.md#github

There are a bunch of extraneous changes in this commit, mostly adding extra files. See our guidance on clear and coherent commits:
https://zulip.readthedocs.io/en/latest/contributing/version-control.html
and take a look at our Git guide:
https://github.com/zulip/zulip-mobile/blob/main/docs/howto/git.md

[chrisbobbe]

Also, have you tested this manually, and does it work as you intend it to? Please also be sure to do that. Thanks! 🙂