Present the correct downsized image dimensions in the Media modal when...

Present the correct downsized image dimensions in the Media modal when inserting. Introduces a context parameter for image_constrain_size_for_editor() instead of relying on is_admin(). props jond3r, nacin. fixes #22738

git-svn-id: https://core.svn.wordpress.org/trunk@23096 1a063a9b-81f0-0310-95a4-ce76da25c4cd