Privacy: Add template tags for building link to privacy policy page.
This introduces the get_the_privacy_policy_link() and the_privacy_policy_link() functions, as well as the privacy_policy_url filter.
A new tests/url/ folder was added to better organize tests related to get_*_url() functions. Previously, those tests were placed in tests/url.php and tests/link/, but neither of those locations are optimal. Placing tests in tests/url.php violates the guideline of creating separate files/classes for each function under test, and using tests/link/ conflates two distinct -- albeit related -- groups of functions. Over time, URL-related tests can be migrated to the new folder.
Props birgire, xkon, azaozz, iandunn.
See #43850.