Display user facing version information in menu

[PIG208]

#1035 is a prerequisite of this.

Figma design here

Planned differences from the Figma design:

• For this issue we won't include information like "up to date". (That would require further discussion to work out just how we want it to work, and how the app would get that information.) Instead we'll only show the version of the running app. See chat thread.

[chrisbobbe]

You have some other PRs currently open; I'd prefer that you focus on those: https://github.com/zulip/zulip-flutter/pulls/Gaurav-Kushwaha-1225

It looks like you're working on a new revision for #1213, and I've just sent a review for #1152.