In `wp_insert_term()`, when no slug is provided, check for an existing term by name. If it exists, use that slug instead of calling `sanitize_title( $name )`.

Prevents creating an endless number of terms like `A+` or `$$$$` in any given taxonomy.

Props wonderboymusic, SergeyBiryukov, aaroncampbell.
Fixes #17689.


git-svn-id: https://develop.svn.wordpress.org/trunk@28733 602fd350-edb4-49c9-b593-d223f7449a82