in status page, separate out issues with pending concerns

[nikomatsakis]

As @cramertj requested in today's @rust-lang/lang meeting, if you have registered a concern on an FCP, it should not be listed in the list of things to do.

Probably the "to do" items should be separated into

• things that you have not checked (also show other people's concerns)
• things where you have concerns

Or .. something?

Similarly, I would like a way to quickly see all outstanding concerns by anyone.