Back buttons

[peteryates]

Currently the output from wizardry_content doesn't include a back link. Back links are part of the design system and should be present on wizards. The guidance states:

Make sure the link takes users to the previous page they were on, in the state they last saw it. Where possible, ensure it works even when JavaScript is not available.

This should be achievable using Rails' link_to(:back).

It probably makes sense for it to be configurable at the wizard level, perhaps having a include_back_links: true option that can be overridden when not needed would make sense.

Refs DFE-Digital/find-a-lost-trn/pull/23