Magic link add link back to login

[jlopes90]

Supersedes https://github.com/codeigniter4/shield/pull/375

[kenjis]

Note, we expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works. If pull requests are not accompanied by relevant tests, they will likely be closed. https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#contributions

If you are not familiar with PHPUnit testing, See Resources

[datamweb]

@jlopes90 Thank you very much for your efforts.