Improve user issue reporting - add copy feature to About popup

[neiljp]

When reporting issues, it is useful to know the current version and platform information. This is already provided in the About popup, so it would be convenient to add a feature to copy that information to the clipboard for use in bug (or feature) reports.

The code already wraps copy functionality, so the primary aspect here is to construct the information appropriately from the data available in the About popup.

We expect to standardize on c for copying, though the command string is somewhat in flux at present. See for example COPY_STREAM_EMAIL.

This was previously discussed in #zulip-terminal>View object has no attribute topic_w on chat.zulip.org.