Introduce `description__like` arg to `get_terms()`. Make `description__like` and `name__like` perform `LIKE`s with a wildcard on both sides of passed string. Previously, strings had to match the beginning of the name, so searching for `burrito` in `This is a burrito` would fail. Adds unit tests.

Props aaroncampbell for the original patch, 5 years ago.
Fixes #8214.



git-svn-id: https://develop.svn.wordpress.org/trunk@25241 602fd350-edb4-49c9-b593-d223f7449a82