PostSettingsMenu: Don't save new posts, reset values on failure

Closes #3158, Closes #3143, Closes #3134
- Added `model.rollback()` when PSM fails to save.
- Added `showErrors` and `showSuccess` helper functions to PSM to abstract
  closing and showing of notifications.
- Added `togglePage` action to indirect the setting of `page`.
- Removed `isStaticPage` property in favor of `togglePage` action
- moved `updateSlug` error catching to outer promise (slugGenerator promise)
- modifying the `page` and `published_at` properties will no longer cause a new post to save
- Close passive notifications on published date parse fail
- Removed promise creation in catch statements

- Changed tests to click on label, rather than the input for
  .post-setting-static-page.