Create a new system panel

We have an ongoing issue with there being too many top level settings panels (see #1090). One way to help with this would be to move Region & Language, Date & Time, Users, and About into a new System panel (mockups).

Previously we had the "details" panel, which served a similar function. We removed that after people seemed to get confused by it, though now I wonder whether it was as much an issue with the "details" name as much as anything else.

Other platforms have a "system" section, so I think people are used to this concept. Also, we've tested the system panel design with users, and it has performed well (see Teams/Design/usability#10 (closed)).