Count all issuable states at once

Instead of doing n queries for n states, do one query to get all the
counts grouped by state, and figure out what the count is for each state
is from that. We can still cache the individual counts (it can't hurt),
but this will help with initial load.

Note that the `opened` scope on `Issuable` includes the `opened` and
`reopened` states, which is why there's a special case.