This is an attempt towards #2684
I am not sure if this is too simplistic for now OR we need to
consider if this will break anyones automation as they maybe using
the output of that command as plain text
Before:
```
▶ pulumi whoami
stack72
```
After:
```
▶ pulumi whoami
User: stack72
Backend URL: https://app.pulumi.com/stack72
```
This simply refactors all the display logic out of the
pkg/backend/filestate package. This helps to gear us up to better unify
this logic between the filestate and httpstate backends.
Furthermore, this really ought to be in its own non-backend,
CLI-specific package, but I'm taking one step at a time here.
