Reactions: Show top N for unencrypted rooms

[jryans]

To avoid showing potentially 100s of reaction types (where a "type" is a single emoji that many may have sent in aggregate, not a single vote) on a single message, we will limit to the top N types.

• For unencrypted rooms, server API will sort and send the top N (this issue)
• For encrypted rooms, client will have to pull all and sort client side (Reactions: Show at most N keys #9570)

The expectation is we'd show the top N types inline with the message, and then add a "view more" disclosure to see the rest if there's more than that.

We're currently thinking N will be 8.

[jryans]

We no longer need different limit approaches for different room types, so we can collapse as duplicate of #9570.