Media: Improve display and accessibility of meta data in detail view.

* Add a `human_readable_duration` function including tests.
* Add 'pixels' after image width/height.
* Add screen reader text for durations.

Props Presskopp, kiranpotphode, milindmore22, stormrockwell, afercia.
Fixes #39667. 


Built from https://develop.svn.wordpress.org/trunk@43633


git-svn-id: https://core.svn.wordpress.org/trunk@43462 1a063a9b-81f0-0310-95a4-ce76da25c4cd