SOCIAL_AUTH_ALLOWED_REDIRECT_HOSTS should allow regexes

[ldeluigi]

Expected behaviour

I'd like to whitelist all the possible subdomains of a domain I trust.

Actual behaviour

Currently this is not possible unless I'm whitelisting all the existing subdomains with SOCIAL_AUTH_ALLOWED_REDIRECT_HOSTS.

Suggested solution

Allow to specify regexes with a settings variable like SOCIAL_AUTH_ALLOWED_REDIRECT_HOST_REGEXES so that I can whitelist \w+.trusteddomain.com