lambda: preserve topic linking

[maxenglander]

It's a shame to lose topic linking. For example, I would like to set a topic like:

USER is on-call for <!subteam^SID>

I can do this by setting sched_name to <!subteam^SID>.

Unfortunately, with the current update_slack_topic logic, this gets turned into:

USER is on-call for <!subteam^SID> | USER is on-call for

This PR changes how the current topic is searched for the first non-linking | symbol in a way that preserves linking.