Media grid details shouldn't use full size image

[nacin]

The media grid details overlay uses the full size image. This can easily be a 10MB photo, which is really, really bad. Instead, it should use the large size image, which is more or less how post.php for attachments works.

If the large size doesn't exist, we can assume one of two things:

• The image is smaller than large, so we can and should use full.
• Someone has deregistered the large size. Let's use full there.

[ocean90]

29498.diff​ works as expected.

[nacin]

In 29704:

Media grid details: Load large images instead of originals, if available.

fixes #29498.