This commit migrates all of our webhooks to use
check_send_webhook_message, except the following:
beeminder: Rishi wanted to wait on this one.
teamcity: This one is slightly more work.
yo: This one is PM-only. I am still trying to decide whether we
should have a force_private argument or something in
check_send_webhook_message.
facebook: No point in migrating this, will be removed as part of
#8433.
slack: Slightly more work too with the `channel_to_topics` feature.
Warrants a longer discussion.
I dug into why we never did this before, and it turns out we did, but
using `$.trim()` (which removes leading whitespace as well!). When
removing the `$.trim()` usage.
Fixes#3294.
