Customize: Introduce Logo support for themes.
Allows a common theme feature to have a common implementation provided by core and available in a consistent location for users. See https://make.wordpress.org/core/2016/02/24/theme-logo-support/
Props kwight, enejb, jeherve, bhubbard, samhotchkiss, zinigor, eliorivero, adamsilverstein, melchoyce, ryan, mikeschroder, westonruter, pento, karmatosed, celloexpressions, obenland. See #33755.