Ability to see 'Issues Missing Estimate' in 'Workload by Assignee' summary

As originally discussed on the related issue JSW-11177.

Per row metrics (assignee) similar to

• Issues
• Issues Missing Estimate
• Original Time Estimate
• Remaining Time Estimate
• Time Spent (tracked at JSW-14771)

"Missing Estimate" is only a count of issues that are missing estimates.
"Remaining Estimate" is not an accurate reflection of remaining work or when exactly we should expect to be completed if everything goes according to the plan (estimate), if there are incomplete issues with remaining estimate = 0

• So any ticket that has not been estimated would fall into that category,
• Plus any ticket that is not in Resolved or Done state, and although it might have had an estimate at some point, but the current remaining estimate = 0

Having an idea of how many such tickets are in the sprint (current or the one being planned) gives us a better assessment of the situation.