gnome-software account UI: Split Sign In vs Register

[sowasred2012]

This issue was originally raised on 19/03/20 at https://github.com/canonical-web-and-design/Ubuntu-design/issues/224 by @matthewpaulthomas - it was decided this repo was a more suitable place for the issue, but for some reason we couldn't just transfer it across, so instead it's been manually recreated.

The gnome-software “Sign In / Register” dialog currently combines the two functions with a single e-mail address field, like Amazon used to:

In review of #222, @cassiocassio suggested that it might be clearer if they’re split out into separate sections.