D8 backports

PR for https://github.com/backdrop/backdrop-issues/issues/106

Issue #1497408 by Dan Silver: Fixed URL being passed through t().
Issue #1431630 by NROTC_Webmaster, tim.plunkett: Clean up API docs for themes directory.
Issue #1454630 by mottihoresh: user_autocomplete() page callback uses as a local variable (coding standards, bad example).