Allow to configure a custom domain

[rixx]

Feature Description

An orga member can configure a custom domain for their pretalx event. The custom domain is then used for

• Links in emails
• Potential URLs in social media tag
• Links in the RSS feed
• Links in frab exports
• Various places in Django settings

[rixx]

This should better wait until I have my PR for andrewgodwin/urlman#4 ready.