Rename PoliciesController#show to PoliciesController#index

What does this MR do?

Noticed in !67900 (comment 647982245) - The PoliciesController list view has a few things that don't make sense for a list view. It's likely that it ended up like this because we used to use this page to show policy configuration options and then we changed it to show a list view plus an "Edit policy management project" button.

• The action should be named #index
• The path should be pluralized

This MR fixes those things and also makes the controller error handling redirect to the list view when there is a problem with the parameters on the #edit action.

This feature is still in development and is behind a feature flag.

Epic for this feature: &5362 (closed)

Screenshots or Screencasts (strongly suggested)

Parameter errors on the edit page now redirect to the list view.

Screen_Recording_2021-08-11_at_12.22.10_PM

Does this MR meet the acceptance criteria?

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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