[#6] Implement Spy::Retries, retrieving Sidekiq Retries data.

This provides data equivalent to the `Sidekiq::Web` Retries page (but
not the linked Retry pages). It uses `Sidekiq::RetrySet` with no limit.
Because of this, the current implementation could experience issues if
there are a large number of retries.

There is no sort applied to the list, so it should reflect any
underlying order returned from Sidekiq.

This is implemented in a similar way to `Spy::Workers`.
[#6] Define Display::Panels::Retries, equivalent to Retries page.

Using the data provided by `Spy::Retries`, define panel
`Display::Panels::Retries`. This displays a list of queued Sidekiq
retries. Note that it is necessary to hook into `#refresh` and call
`Spy::Retries#refresh`. The list is defined to be precisely as many
table rows long as the panel minus the header row.

Mount a retries panel in `Display::Screen` in the `:main` panel,
binding to keypress `<r>`. Add a Retries entry to the `:header` panel
menu.

Note that the `Sidekiq::Web` Retry page (linked from Retries) is not
implemented as part of this work.
Merge branch 'panel-retries'