ReaderPost performs double-save on the ContextManager when saved

[dvdchr]

Refs #19765 (comment)

Currently, when saving a post in the Reader, a didSave hook is implemented on ReaderPost to update the parent object, ReaderCard:

WordPress-iOS/WordPress/Classes/Models/ReaderPost.m

Lines 513 to 522 in be75d66

	- (void) didSave {
	[super didSave];
	// A ReaderCard can have either a post, or a list of topics, but not both.
	// Since this card has a post, we can confidently set `topics` to NULL.
	if ([self respondsToSelector:@selector(card)] && self.card.count > 0) {
	self.card.allObjects[0].topics = NULL;
	[[ContextManager sharedInstance] saveContext:self.managedObjectContext];
	}
	}

As quoted above, the didSave method made another save call to ContextManager, effectively double-saving every time a post is saved. This can make Core Data operations slightly less visible, and might introduce side effects (i.e., the issue described in #19765). As an idea, we can look into moving this part of the logic to the post-saving logic described in #19765 (review).