Leverage `size_format()` when displaying `max_upload_size`: 
* In `media_upload_form()` 
* The `uploader-inline` Underscore template in the media modal. 

Replaces 2 buggy inline implementations that could display size units off by one, or size values off by one.

Fixes #25643.



git-svn-id: https://develop.svn.wordpress.org/trunk@28343 602fd350-edb4-49c9-b593-d223f7449a82