Relax validation of source field

[pmac]

When testing locally, or even sometimes when users submit from around the web, the subscription request will be rejected by CTMS because their source data isn't a fully valid URL. It will fail if there is not a protocol, or if it's using http://localhost in some local testing, or if it's simply a path instead of a full URI. The data that comes in for that field (in basket it is source_url) is supposed to be a URL, but it is okay for it to be other things. It's meant as a way for us to identify the source of subscriptions if the site is willing to send us anything. I believe we should avoid rejecting any submission based on this data being not a perfectly valid URL. I recommend we remove all URL validation from this field and treat it simply as a text field and truncate it if it's too long.