fixes https://github.com/TryGhost/Team/issues/777
- the unsaved settings modal would come back if you left a settings
sub-page with unsaved changes and then came back to the page
- if you then clicked on "Leave", Admin would crash with a Sentry error
of `Cannot read property 'finally' of undefined`
- we had a similar bug in
39c850f4fe
and the fix was to remove the `.finally` because it assumed the
confirm action returned a promise
- this commit performs a similar fix and seems to resolve the problem
no issue
- upgrade to latest `ember-source` and related dependencies including `ember-cli`
- upgrade to latest `ember-mocha` and modern ember testing setup
- https://github.com/emberjs/rfcs/blob/master/text/0268-acceptance-testing-refactor.md
- switch from using global acceptance test helpers and `native-dom-helpers` to using the new `ember-test-helpers` methods
- use [`chai-dom`](https://github.com/nathanboktae/chai-dom) assertions where in some places (still a lot of places in the tests that could use these)
- pin `ember-in-viewport` to 3.0.x to work around incompatibilities between different versions used in `ember-light-table`, `ember-infinity`, and `ember-sticky-element`
- incompatibilities manifested as "Invalid value used as weak map key" errors thrown when using `ember-light-table` (subscribers screen)
- pin `ember-power-datepicker` to unreleased version that contains a move from global acceptance test helpers to modern test helpers
closesTryGhost/Ghost#8483
- Added a new modal component that gets rendered when leaving general/settings after changes have been done but not saved
- Removed independent saving logic for social URL for consistent UX
