consider adding counter records

[madhums]

We display counters in many places

• story updates count in the stories listing page
• discussions count in the story detail page
• posts count in the discussion detail page

AR gives a nice counter_cache option which we can use. Consider this?

[madhums]

Since we use discard and the native counter_cache doesn't support it, perhaps consider https://github.com/magnusvk/counter_culture/

[madhums]

And since we also use Mobility, we might definitely bump in to magnusvk/counter_culture#344