Hide staffing requests / silence messages during backlogs

[marcua]

• We wrote this task before implementing the autostaff logic. We'd want to research the need for this after deployment. *

• Right now, StaffBot goes down the list of experts by priority and sends Staffing requests. This optimized for latency: if a project isn't staffed, the bot keeps reaching out to new experts.

• When we have backlogs, Available Tasks gets filled and it's hard to complete older or higher-priority tasks.

• [ ] Stop messaging experts that a specific new task is available if we have more than X tasks of that type available. Still create staffing requests---experts will just get a more generic "work is available for you" message every Y hours.
• [ ] Only show the top Z tasks of each type in Available Tasks, sorted first by project priority and second by age of staffing request.