Introduce some new template functions for archive titles and descriptions:

 * `get_the_archive_title()` and `the_archive_title()` for returning/displaying the title of the current term, date, post type, post format, or author archive.
 * `get_the_archive_description()` and `the_archive_description()` for returning/displaying the description associated with the current term archive.

Fixes #21995
Props obenland, DrewAPicture


git-svn-id: https://develop.svn.wordpress.org/trunk@30223 602fd350-edb4-49c9-b593-d223f7449a82