Bug 824433 - Bookmarks backup takes a long time to write out on shutdown. r=mano, a=sledru

Includes regression fixes from Bug 971686 and Bug 972434
Bug 967168 - DevTools Addon icons should not be inverted on light theme. r=jwalker, a=sledru
Bug 971426 - Define IMPL_MFBT when building standalone js. r=gps, a=NPOTB
Bug 971276 - Hitting enter should insert the currently selected item from an autocomplete list in the Style Editor. r=harth, a=sledru
Bug 966384 - Promises on workers use correct busy count. r=khuey, a=sledru