Update login page background responsiveness

[andybraren]

Definitely not perfect, but fixes a few things for now:

• Swaps PNG for a crystal clear SVG
• Switches background positioning to contain to always keep Lemmy in view and never stretched
• Removes height-based width declaration (min-width: 100vh) to fix horizontal scrollbars from showing

Below I've also included what this would look like with the updated background image of #283, which removes the hard corners. Might tweak that a bit more.

cc @aevo98765

Before

Roughly approximating what's seen on a vertical monitor, a floating window, and a full 16:9 window.

After

Or with the new background image of #283:

[vishnoianil]

@andybraren can you please cherry-pick to release-1.0 branch as well.