* Add arrows back to edit-all toggle button
* Don't show dimensions if the item being inserted doesn't have them.
* Set pop-up windows to be a percentage of the window height for better
use of space
* Fix broken fieldholdersmall styling within files edit
* Add placeholder if no preview
*Take out minFileSize so upload will work on Android phones
1. Refactored some gridfield styles to enable reuse (created a mixin
file to hold mixins that generate specific css objects/elements).
2. Made some scss comments use // so they wouldn't appear in generated
css
Fixed the calculation of height for the upload (non AssetUploads) area
so that the save button would show. Took off the min-height, as this
will cause UI issues when there are multiple files open for editing.
