user invites: web: cleanup cloud signup page & fix mobile/small display

Created by: slimsag

This change does 3 things:

1. Cleans up the negative messy HTML/CSS previously used on this page (useless divs, negative margins, fixed positioning, etc.)
2. Fixes mobile displays
3. Fixes small/medium displays

No effective change on regular/laptop display sizes.

Fixes mobile displays

iPhone 11 pro, before:

after:

Bottom of page before | after

Fixes small/medium displays

Before:

After:

Signed-off-by: Stephen Gutekanst [email protected]