Aggressively restrict projects when filtering by assignee or author
See #24201 (comment 19433090) and below.
We should be able to improve the dashboard's performance by special-casing the
IssuableFinder when filtering by author or assignee ID:
- Currently we find every project the user can see, and then figure out which have issuables they can see that are authored or assigned by the passed user ID.
- Instead, we can look for all issuables that are authored or assigned by the passed user ID, and then figure out which ones are in projects the current user ID can see.
GitLab.com has a lot of public projects, so any change to this should be noticeable.