10 Reaction Limit Erroneously Prevents incrementing Existing Reaction

## Issue Description
The check that prevents users from adding more than 10 types of reactions on a post also prevents a user from incrementing an existing type of reaction. I cannot react to the post at all, even on existing reaction types.

https://github.com/humanmade/H2/blob/695cb2a1f5ec3834d0608d0bc6ca4976e489761a/src/plugins/reactions/Reactions.js#L166-L169

## Steps to reproduce:
1. Navigate to https://updates.hmn.md/2019/09/17/introducing/
1. Attempt to increment an existing reaction on the post (the wine glass, for example)
1. Observe the error
1. Observe, also, that the "add new reaction" icon is still available



## Expected Result:
I should be able to increment a current reaction type without error. I would expect to only see the error seen above if I were trying to add a new type of emoji reaction.

Additionally, I should not see the "add new reaction" icon if the maximum has been reached.