Administration: Bring some consistency to handling list table view modes.

* Remove duplicate variables and DocBlocks.
* Add missing description for the `$mode` global.
* Use sentence case for "Compact view" and "Extended view" labels.

Follow-up to [48398], [48423].

Props afercia, Offereins, SergeyBiryukov.
See #49715.
Built from https://develop.svn.wordpress.org/trunk@48424


git-svn-id: https://core.svn.wordpress.org/trunk@48193 1a063a9b-81f0-0310-95a4-ce76da25c4cd