refs #6039, closes #6047, closes #6048

Implements the error handling refactor as outlined in #6048 (comment)

• add failing tests for Regression: Not redirected to sign-in with invalid session #6047 & Regression: No re-auth modal when saving editor with invalid session #6048
• add 401 error handling to application route to redirect to signin screen when user has invalid session
• remove .catch() in ValidationEngine.save so that we let errors bubble up
• review all uses of .save().catch() and remove or update to let errors bubble if appropriate
• display the re-auth modal if user's session is invalid when attempting to save a post in the editor
• use application route's error action to display an appropriate alert if not previously handled