Increment an existing reaction
This allows you to increment an existing reaction below a message by clicking on
it.
At the moment, this is not linked to the action bar, so they each are using
local state. We'll likely want to add some mechanism so that we can local echo
to both of these UI areas at the same time, but that can be done separately.
Fixes https://github.com/vector-im/riot-web/issues/9486
