Replaces all uses of `TEMPLATEPATH` and `STYLESHEETPATH` in core with `get_template_directory()` and `get_stylesheet_directory()`.

Add `@deprecated` annotations to `TEMPLATEPATH` and `STYLESHEETPATH` definitions.

Props obenland, aaroncampbell. 
Fixes #18298.


git-svn-id: https://develop.svn.wordpress.org/trunk@28563 602fd350-edb4-49c9-b593-d223f7449a82