Add titles and furls to sites

Sites can already have homepages which differ from their owning organisation's homepage. Given that the furl attribute is simply a shorter form of the full URL for a site, it follows that they have furls too.

We are currently inconsistent with how we apply the (new) homepage for a site.
  * If you look at the site in Transition, it says the new homepage is the one
    specified for that site in Redirector.
  * If you visit the old homepage, you get redirected to the site's new
    homepage, which is correct.
  * However, if you visit a page with an archive/unresolved mapping, it links
    to the organisation's new homepage, which is unhelpful or even
    inappropriate.

Therefore, we should change Bouncer's archive behaviour so that it links to the site's new homepage. To make it consistent, we then need to make it use the furl (if any) and a title which describes the site, rather than the organisation. The site's title might often be the same as the organisation's name.

With a related PR in Bouncer, this is an alternative to https://github.com/alphagov/bouncer/pull/81 
This is confusing to me in the context of configuring a site - is the title for the old site or the new homepage? We have a site YAML file per old site, but have fields in it which hold data about the old site, new site, or the (current... UKVI/UKBA) owning organisation without the field names being clear about what they refer to.

Since we probably need to clear out all existing data for site titles and furls and start again before deploying this, can we use more precise names so that their meaning is clear without having to understand Transition's current import process? For example `new_site_title` and `new_site_furl`. Site config currently requires a lot of background knowledge, and we should be trying to reduce the need for that wherever possible.