Default SD 2.x models to 768x768

[ndahlquist]

Stable Diffusion 1.x is trained for images of size 512x512.
Stable Diffusion 2.x is trained for images images of size 768x768.

Therefore, I propose we default all models based on SD 2.x to 768x768. This will result in higher quality images for these models.

From a personal perspective, I also think this is important because there is a popular narrative that SD 2.x is worse quality than 1.x. Generating images of the wrong size doesn't help with that 😆

Important detail: only logged in users can request images of this size, so we should prompt for login when selecting one of these models.