Expose Rail rates vs Rail rate change queue

Currently, on the rail details page, there's a section "Rate Change Queue". It's empty when there has been no rate change, even if the rail currently has a rate:

On rails with rate changes, it shows them:

This is not intuitive, as the user isn't thinking about changes in rates, they are thinking about _rates_. If the rail currently has a rate, it should show up in this table. Then, we can also remove the "Payment rate" tile from metrics.

Start at the rail details page and trace how the “Rate Change Queue” table is populated and how the “Payment rate” metrics tile is rendered. The work is done when a rail’s current rate appears in the table even without a rate change, and the Payment rate tile is removed.

Written by the indexing model from the issue text.