Background Sync

[theSdev]

Per https://mozilla.github.io/standards-positions/#background-sync Mozilla considers Background Sync "Harmful" but https://platform-status.mozilla.org/#background-sync shows it as "Developing" which isn't true anymore.

[chrisdavidmills]

@theSdev I've changed it to "under-consideration" — there is an extensive bug, and we've considered it quite a lot, but we don't like it in it's current state. Does that seem fair? The only other option is really "not-planned", which doesn't sound right.