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

Closes #3158, Closes #3143, Closes #3134
- Added `togglePage` action to indirect the setting of `page`. togglePage resets page back to its original value in the case of an error saving.
- Removed `isStaticPage` property in favor of `togglePage` action
- `updateSlug` and `updatePublishedAt` similarly reset their respective properties on failure
- 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
Just a heads-up that `togglePage` should be clearing notifications and is going to need a `this.notifications.closePassive()` in the `.then` and `.catch` handlers of the `.save`.

See #3181.