Display existing issue to user after Sentry logs error

Often when people see UI break, there are frontend errors in the console. Occasionally we show a flash message.

When we log the error to Sentry, we could check for existing Issues. If there is one, show a notification that links to the existing issue.

This can save a lot of 'is this known?' discussions in Slack, and/or the subsequent write-up of an issue that you later realise is a duplicate

Fake example using window.Flash: