Display pending request count for each request type

[matthew-li]

• For each request type under "Admin" (for superusers) or "Staff" (for staff users) > Requests, display a bubble/alert containing the number of pending requests of that type (similar to a notification count on a phone).
• Display the sum S of these counts next to the "Requests" list item and next to the "All Requests" sub-list item.
• Also display S next to the "Admin" or "Staff" menu item.

(We actually do something similar already on: Admin > Requests > All Requests. Using the same icon/color, but with just the number, might be okay.).

[matthew-li]

We'll think about using Redis to cache request counts so they're not re-computed on each page load. Example in #506.