Redirect to new event detail URL when slug changes

[saerdnaer]

For FOSDEM 2016 the URL of an Event detail page is structured as follows:

• https://fosdem.org/2026/schedule/event/J8UCRH-why_i_volunteer_at_fosdem_and_you_should_too/
• the slug is J8UCRH-why_i_volunteer_at_fosdem_and_you_should_too
• where J8UCRH is random code generated by Pretalx, and the rest of the slug is initially generated from the title

When the slug changes, the URL 404s – we should change this to a redirect to the new URL (which can be found by searching for all slugs starting with J8UCRH.