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
refs https://github.com/TryGhost/Ghost/issues/9724
- `formatMarkdown` was previously changed to return a SafeString but that meant any direct usage of the helper had to account for not dealing with a basic String type
- changed `formatMarkdown` to return a basic String
- modified helper usage of `{{sanitize-html}}` to use triple-curlies
refs https://github.com/TryGhost/Ghost/issues/9724
- extract html sanitisation into a Koenig helper `{{sanitise-html}}` (all markdown handling will eventually move into Koenig too)
- render sanitised html in the html card
