Adds previous_url method to url_helpers

[colinkey]

Purpose

Adds the previous_url helper as described in #1025

Description

If the Referer header is set, this method should return the url from the header. A user should pass a Lucky::Action or Lucky::RouteHelper as a fallback in instances where the Referer header is not set.

Functionally, it works very similar to the redirect_back method added in #1168 but is suitable to use within a Lucky::Page

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[matthewmcgarvey]

Nice work 🎉

[jwoertink]

🙌 perfect! Thanks so much for contributing 😄