Support both integer and string as epic count key

What does this MR do?

Epic state uses enum for saving state (other issuables use string), because of this inconsistency we override count_key which should return state as string converted to symbol (.e.g. :closed). In rails 5 these integer states are automatically converted to strings by Active Record.

But if subquery is used (to optimize searching), Active Record doesn't convert state into string automatically (probably it can not deal with the more complex query).

This means that epic state can be either integer (if subquery is used) or string (if AR converted state to string automatically), so in count_key we should convert state into string only if it's an integer.

What are the relevant issue numbers?

Closes #8824 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
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