Create a stopgap warning

[roed]

Sometimes issues can produce mathematically incorrect results, and we should warn users of the problem. If the issue is fixed promptly then nothing more needs to be done, but sometimes tickets remain unresolved for months or years (c.f. #11358). This ticket adds a function that makes it easy to create a second ticket warning users of the problem while it's being resolved.

On trac, this functionality will be supported by the Stopgaps field. Create a new ticket that uses the stopgap function and include a link to it in the Stopgaps field in the original ticket.