Reactions: Show at most N keys

[jryans]

To avoid showing potentially 100s of reaction keys (where a "key" is a single emoji that many may have sent in aggregate, not a single vote) on a single message, we will limit to at most N keys. (This is intentionally written as just a limit without any sorting implied. See https://github.com/vector-im/riot-web/issues/9698 for sorting.)

For all rooms (unencrypted and encrypted), the client will show at most N keys 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.