We now find all (possibly) relevant service bots for a message
in the call to get_recipient_info. This allows us to eliminate
some code that would patch them after we rendered.
The get_service_bot_events() function will ignore any service
bots that weren't actually mentioned in the message (due to
backticks) or part of the active user ids.
There are several reasons to extract this function:
* It's easy to unit test without extensive mocking.
* It will show up when we profile code.
* It is something that you can mostly ignore for
most messages.
The main reason to extract this, though, is that we are about
to do some fairly complex splicing of data for the use case
of mentioning service bots on streams they are not subscribed to,
and we want to localize the complexity.