refs https://github.com/TryGhost/Ghost/issues/6976
- adds custom `MaintenanceError` and associated error checking functions
- updates app route and notifications service to handle `503` errors via the `upgrade-status` service
refs https://github.com/TryGhost/Ghost/issues/6949
Handle version mismatch errors by:
- displaying an alert asking the user to copy any data and refresh
- disabling navigation so that unsaved data is not accidentally lost
Detailed changes:
- add `error` action to application route for global route-based error handling
- remove 404-handler mixin, move logic into app route error handler
- update `.catch` in validation-engine so that promises are rejected with the
original error objects
- add `VersionMismatchError` and `isVersionMismatchError` to ajax service
- add `upgrade-status` service
- has a method to trigger the alert and toggle the "upgrade required" mode
- is injected into all routes by default so that it can be checked before
transitioning
- add `Route` override
- updates the `willTransition` hook to check the `upgrade-status` service
and abort the transition if we're in "upgrade required" mode
- update notifications `showAPIError` method to handle version mismatch errors
- update any areas where we were catching ajax errors manually so that the
version mismatch error handling is obeyed
- fix redirect tests in editor acceptance test
- fix mirage's handling of 404s for unknown posts in get post requests
- adjust alert z-index to to appear above modal backgrounds
