refs https://github.com/TryGhost/Ghost/issues/7865
- updates all settings screens to use EC tasks and `gh-task-button` to get save-state save buttons
- removes now-unused `settings-save` mixin
- moves the order of button color CSS so that grey buttons can change to green/red after completing
- removes the heading from `apps-loading` template so that there's no odd flash when loading slack/amp screens directly