Adds a "Discard all" button to queue show page, if there are enqueued jobs

[matthewbjones]

Adds a Discard all button when viewing a specific queue, if that queue has any enqueued jobs.

[matthewbjones]

@rosa I tried to have the "discard all jobs" test (https://github.com/rails/mission_control-jobs/pull/218/files#diff-8a8a64e1cafc6bc8a206a4f417ebddabb796e7169c0f83cb43d58a7077ba2671R11) assert that after clicking and confirming the Discard all button, that the queue show page would have 0 rows (like https://github.com/rails/mission_control-jobs/blob/main/test/system/discard_jobs_test.rb#L21) and also assert_no_text "Discard all"

However, the code blows up at https://github.com/rails/mission_control-jobs/blob/main/app/controllers/mission_control/jobs/queues_controller.rb#L14 because it seems like ActiveJob.queues no longer has the queue in it after all the jobs were discarded (a side effect of the test suite using Redis underneath?) This also prevents https://github.com/rails/mission_control-jobs/pull/218/files#diff-e99044e9fdb36c40c50af4dd65a95a136150cd6b74e7716c4463e0e64398b198R7 from using redirect_back, as the queue show page throws an exception, because the queue is suddenly missing.

I figured I would commit what I have and seek your thoughts.