Issue #1507896 by b0unty, tuutti, Jeff Burnz, LewisNyman, tameeshb,...

Issue #1507896 by b0unty, tuutti, Jeff Burnz, LewisNyman, tameeshb, realityloop, cbanman, umarzaffer, manumilou, mgifford, mducharme, oakulm, lauriii, alexpott, joelpittet, MaskyS, markcarver, xjm: Allow theme developers to add the default logo filename to the theme's .info.yml