Hotfix/mailing list

[VirginiaDooley]

Users receive a 500 error when trying to sign up for the mailing list.
Closes https://democracy-club-gp.sentry.io/share/issue/f2465a9aa44148678fbedd491696006d/
Ref https://trello.com/c/umVpqIL6/3345-fix-mailing-list-signup

This work refactors definition of the source_url ensuring it's not blank. This has been installed locally in Website with success.

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1. References a specific issue or if not, describes the bug or feature in detail
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[VirginiaDooley]

Fix mailing list signup

[Bekabyx]

No longer getting an error on this locally, looks like it's sorted :)