Don't default to an unselectable image size

[tellyworth]

#7520 [8653] lets you specify a default image size. If you set a default of 'medium', and find an image in the media library tab that doesn't have an intermediate of that size (a very small image for example), the 'medium' choice will be selected even though it is disabled. Same for 'large'.

The enclosed patch makes it skip the disabled sizes and choose the next largest available one (usually 'full', which is always available).

[westi]

2.6.1 has been released, moving to 2.6.2 milestone

[ryan]

(In [8670]) Don't default to an unselectable image size. Props tellyworth. fixes #7528 for trunk