Add issue limit warning when configuring issue source

Summary

As described in Working With Plans there is a 5000 issue limit to what a Plan is capable of handling. Despite this limit, users can still include issues way beyond this limit when configuring the Issue source which will ultimately lead to performance problems.

Propose solution

To avoid the unexpected surprise of performance problems in Advanced Roadmap and also make users aware of this limit, a warning should be displayed in the Confirm what's in scope step. This warning can either always be visible as a banner or only trigger when more than the acceptable limit has been selected.