organisation: Truncate person descriptions in search results
organisation: Raise char limit on YouTube identifiers

This looks to have been a copy-paste error from the Twitter field.
YouTube user identifiers can be (and often are) longer than 18 chars.
organisation: Convert youtube field to youtube_url

It's a bit more complicated than just using a user id in the way we have
up until now. Some users have channels which live at

    https://www.youtube.com/channel/foo

rather than

    https://www.youtube.com/user/foo

so instead convert this field into a URL field.