Add a per-class delete button for mixed queues

#1969 adds a Delete button to each row of a queue's pending-jobs page. It deliberately stops there. The case it does not cover is a mixed-class queue where you want to drop one class and keep the others.

Neither existing option fits that:

• Remove Queue takes everything, regardless of class. It is very cheap — one pipelined SREM plus UNLINK, constant work whatever the queue depth — so it is the right tool when you want the queue gone, and usually the right answer in practice.
• The per-row button from #1969 deletes by exact payload, which is fine for a handful of rows and unusable for thousands.

Why this is blocked rather than just unwritten

The obvious implementation is Job.destroy(queue, klass) with no args, and that is the expensive branch:

data_store.everything_in_queue(queue).each do |string|   # LRANGE 0 -1, whole queue into a Ruby array
  if decode(string)['class'] == klass
    destroyed += data_store.remove_from_queue(queue, string).to_i   # O(N) LREM, once per match
  end
end

Every payload in the queue gets loaded into memory, then each match costs a linear scan of the list. Putting that behind a one-click button in the web UI turns a deep queue into an incident. That is a much worse shape than Remove Queue, which is what an operator would otherwise reach for.

So this should wait on a destroy that does not load the queue into memory — #1788 (nevans) is aiming at exactly that. Once a bounded-memory destroy exists, a per-class button becomes a small, safe addition rather than a foot-gun.

Shape, when it is unblocked

• Something like Job.list_job_classes(queue) to render classes with counts. #1732 (h3nnn4n) had a version of this; worth borrowing with credit.
• A delete control per class, on the queue page.
• Confirmation, since it is destructive and broader than the per-row button.
• Worth deciding whether it unwraps ActiveJob::QueueAdapters::ResqueAdapter::JobWrapper so an Active Job shop sees real job classes instead of one JobWrapper row — #1732 did, and it is still the correct constant. Related to #1968.

Related: #1969 (per-row deletion), #1788 (low-memory destroy, the blocker), #1732 (original bundled attempt), #1968.