Refactor new->edit transition to avoid editor re-renders

[kevinansfield]

closes TryGhost/Ghost#8287
closes TryGhost/Ghost#8750

• moved all editor model functionality into the new editor route+controller
• moved editor template from editor/edit.hbs to editor.hbs
• refactored editor controller and the editor.new/editor.edit routes to work with a persistent editor rather than a teardown/re-render when transitioning from new->edit
• fixed issue in {{gh-markdown-editor}} for autofocus behaviour when the component isn't re-rendered
• fixed issue in {{gh-simplemde}} that was causing multiple updates to the value property when a new value is passed in to the component
• added {{gh-scheduled-post-countdown}} component to lower the noise in the editor controller
• removed editor route/controller mixins
• removed the editor.edit and editor.new controllers

TODOs

• re-implement loading behaviour for the edit route

[coveralls]

Coverage decreased (-0.01%) to 71.637% when pulling 39a2040 on kevinansfield:refactor-new-edit-transition into 8b3c92e on TryGhost:master.

[coveralls]

Coverage increased (+0.003%) to 71.655% when pulling afbb4d5 on kevinansfield:refactor-new-edit-transition into 8b3c92e on TryGhost:master.

[coveralls]

Coverage increased (+0.003%) to 71.655% when pulling 2ce3f57 on kevinansfield:refactor-new-edit-transition into 5a54a60 on TryGhost:master.

[kirrg001]

When i add a new post and select the title, then type and open the PSM, the PSM closes very fast and auto-saves. Expected? 🤔

[kevinansfield]

Yes, that's the same as the current behaviour. Maybe something we can open a separate issue for if it's a pain.

[kevinansfield]

Not sure why Travis says it failed, all the tests passed and the coverage report was triggered! Going to merge as-is because it was ✅ and nothing changed apart from commits being squashed.