Theme Directory - theme screenshot on tablet view

[ramiy]

In single the page, the theme screenshot, on tablet view, reduced to 571 pixels. Why?

On tablets, @media only screen and (max-width: 782px), the screenshot should not have max-width: 571px;. It should be full-width (up to 684 pixels).

On mobile and desktop we don't reduce the image size with max-width.

[obenland]

In 6326:

Themes: Don't restrict screenshot on tablets.

Props ramiy.
Fixes #1277.